Auto detect background image (#7849)

##  Summary of the Pull Request
Added watch on desktopImagePath to check when the path equals "DesktopWallpaper"
If it does equal "DesktopWallpaper" it replaces the path with a path to the desktop's wallpaper

*I am a student and this is my first pull request for Terminal so please give feedback no matter how small. It's the best way I can learn.

## PR Checklist
* [X] Closes #7295 
* [X] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [?] Tests added/passed
* [X] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: MicrosoftDocs/terminal#155
* [?] Schema updated. (Not sure if this is needed, also not sure where this would be)
* [X] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #7295 (Have only talked with the people on the issue, which I don't think has any core contributors)

## Detailed Description of the Pull Request / Additional comments
I am using SystemParametersInfo for SPI_GETDESKWALLPAPER which puts the path into a WCHAR and that is then inserted as the BackgroundImagePath.

I do not think an additional test would add value. The SPI_GETDESKTOPWALLPAPER uses the computers local wallpaper path and puts it into a WCHAR, which then I feed into BackgroundImagePath() as it's new path. I don't think there adds value in making a static path of the desktop background and testing that, given that static tests are already done for "BackgroundImage()".

## Validation Steps Performed

(Manual Validation - Test False Value)
1. Ran Terminal
2. Set setting ["backgroundImage": "<some random img path>"] under profiles->defaults
3. Verified terminal's background is not the desktops wallpaper. 

(Manual Validation - Test True Value)
1. Ran Terminal
2. Set setting ["backgroundImage": "DesktopWallpaper"] under profiles->defaults
3. Verified the background image matches the desktop background image. 

(Manual Validation - Multiple Tabs True Value)
1. Ran Terminal
2. Set setting ["backgroundImage": "DesktopWallpaper"] under profiles->defaults
3. Verified the background image matches the desktop background image.  
4. Opened new tabs
5. Verified the background image matches the desktop background image for each tab.

.github/actions/spell-check/dictionary/apis.txt

@@ -45,6 +45,8 @@ RSHIFT
 rx
 serializer
 SIZENS
+GETDESKWALLPAPER
+UPDATEINIFILE
 spsc
 STDCPP
 syscall

src/cascadia/LocalTests_SettingsModel/DeserializationTests.cpp

@@ -55,6 +55,7 @@ namespace SettingsModelLocalTests
         TEST_METHOD(TestHelperFunctions);
 
         TEST_METHOD(TestProfileBackgroundImageWithEnvVar);
+        TEST_METHOD(TestProfileBackgroundImageWithDesktopWallpaper);
 
         TEST_METHOD(TestCloseOnExitParsing);
         TEST_METHOD(TestCloseOnExitCompatibilityShim);
@@ -1400,6 +1401,28 @@ namespace SettingsModelLocalTests
         VERIFY_ARE_NOT_EQUAL(0u, settings->_profiles.Size());
         VERIFY_ARE_EQUAL(expectedPath, settings->_profiles.GetAt(0).ExpandedBackgroundImagePath());
     }
+    void DeserializationTests::TestProfileBackgroundImageWithDesktopWallpaper()
+    {
+        const winrt::hstring expectedBackgroundImagePath{ winrt::to_hstring("DesktopWallpaper") };
+
+        const std::string settingsJson{ R"(
+        {
+            "profiles": [
+                {
+                    "name": "profile0",
+                    "backgroundImage": "DesktopWallpaper"
+                }
+            ]
+        })" };
+
+        VerifyParseSucceeded(settingsJson);
+
+        auto settings = winrt::make_self<implementation::CascadiaSettings>();
+        settings->_ParseJsonString(settingsJson, false);
+        settings->LayerJson(settings->_userSettings);
+        VERIFY_ARE_EQUAL(expectedBackgroundImagePath, settings->_profiles.GetAt(0).BackgroundImagePath());
+        VERIFY_ARE_NOT_EQUAL(expectedBackgroundImagePath, settings->_profiles.GetAt(0).ExpandedBackgroundImagePath());
+    }
     void DeserializationTests::TestCloseOnExitParsing()
     {
         const std::string settingsJson{ R"(

src/cascadia/TerminalSettingsModel/Profile.cpp

@@ -58,6 +58,8 @@ static constexpr std::string_view RetroTerminalEffectKey{ "experimental.retroTer
 static constexpr std::string_view AntialiasingModeKey{ "antialiasingMode" };
 static constexpr std::string_view TabColorKey{ "tabColor" };
 
+static const winrt::hstring DesktopWallpaperEnum{ L"DesktopWallpaper" };
+
 Profile::Profile()
 {
 }
@@ -244,17 +246,38 @@ void Profile::LayerJson(const Json::Value& json)
 }
 
 // Method Description:
+// - Either Returns this profile's background image path, if one is set, expanding
 // - Returns this profile's background image path, if one is set, expanding
 //   any environment variables in the path, if there are any.
+// - Or if "DesktopWallpaper" is set, then gets the path to the desktops wallpaper.
 // Return Value:
-// - This profile's expanded background image path / the empty string.
+// - This profile's expanded background image path / desktops's wallpaper path /the empty string.
 winrt::hstring Profile::ExpandedBackgroundImagePath() const
 {
     if (_BackgroundImagePath.empty())
     {
         return _BackgroundImagePath;
     }
-    return winrt::hstring{ wil::ExpandEnvironmentStringsW<std::wstring>(_BackgroundImagePath.c_str()) };
+    // checks if the user would like to copy their desktop wallpaper
+    // if so, replaces the path with the desktop wallpaper's path
+    else if (_BackgroundImagePath == to_hstring(DesktopWallpaperEnum))
+    {
+        WCHAR desktopWallpaper[MAX_PATH];
+
+        // "The returned string will not exceed MAX_PATH characters" as of 2020
+        if (SystemParametersInfo(SPI_GETDESKWALLPAPER, MAX_PATH, desktopWallpaper, SPIF_UPDATEINIFILE))
+        {
+            return winrt::hstring{ (desktopWallpaper) };
+        }
+        else
+        {
+            return winrt::hstring{ L"" };
+        }
+    }
+    else
+    {
+        return winrt::hstring{ wil::ExpandEnvironmentStringsW<std::wstring>(_BackgroundImagePath.c_str()) };
+    }
 }
 
 winrt::hstring Profile::EvaluatedStartingDirectory() const