Add post type to editor

[koke]

To be able to show the Page Templates selector only on pages, we need to differentiate between posts and pages. Currently we are hardcoding a postType of post in all cases.

This PR adds the necessary structure for the apps to inject that post type into Gutenberg, but the changes to the apps to actually do that will come in future PRs.

I have changed the bridge code in a way that would be compatible with the existing app implementations (it would just default to post), so these should be fine to merge before we make changes to the apps.

Part of #1463. The proper feature flag is coming up as part of #1576, and we'll need PRs in both apps to finish that issue.

To test:

From WPAndroid, hardcode the "page" postType, or try with the approach in wordpress-mobile/WordPress-Android#10775:

diff --git a/libs/editor/WordPressEditor/src/main/java/org/wordpress/android/editor/GutenbergContainerFragment.java b/libs/editor/WordPressEditor/src/main/java/org/wordpress/android/editor/GutenbergContainerFragment.java
index 2d874b7105..1549e391cf 100644
--- a/libs/editor/WordPressEditor/src/main/java/org/wordpress/android/editor/GutenbergContainerFragment.java
+++ b/libs/editor/WordPressEditor/src/main/java/org/wordpress/android/editor/GutenbergContainerFragment.java
@@ -79,4 +79,5 @@ public class GutenbergContainerFragment extends Fragment {
                 BuildConfig.DEBUG,
                 BuildConfig.BUILD_GUTENBERG_FROM_SOURCE,
+                "page",
                 isNewPost,
                 localeString,

From WPiOS: wordpress-mobile/WordPress-iOS@c7885da

Then check with the JS debugger that the editor is initialized with the right post type.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[pinarol]

This is looking pretty good. I was able to verify that postType is passed properly with WPiOS but couldn't get WPAndroid PR working, it seems like it needs update from develop. But we can handle that separately.

[pinarol]

small note, I had to change

 func gutenbergPostType() -> String? {
         return post is Page ? "page" : "post"
     }

into

 func gutenbergPostType() -> String {
         return post is Page ? "page" : "post"
 }

on WPiOS side, it was just giving a warning but the method was not getting called.