Only add ! to selector class matching template candidate

Fixes #7226.

Before this PR, if you had a class like:

```css
.one .two {
  background: black
}
```

...and then used `!one` in your template, the generated CSS would be this:

```css
.\!one .\!two {
  background: black !important
}
```

This would cause the styles to not be applied unless you also added `!` to the beginning of other classes in the template that are part of this selector.

This PR makes sure that other classes in the selector aren't mistakenly prefixed with `!`, so that you can add `!` to only one of the classes in your template and get the expected result.

src/lib/generateRules.js

@@ -88,7 +88,7 @@ function applyPrefix(matches, context) {
   return matches
 }
 
-function applyImportant(matches) {
+function applyImportant(matches, classCandidate) {
   if (matches.length === 0) {
     return matches
   }
@@ -98,7 +98,10 @@ function applyImportant(matches) {
     let container = postcss.root({ nodes: [rule.clone()] })
     container.walkRules((r) => {
       r.selector = updateAllClasses(r.selector, (className) => {
-        return `!${className}`
+        if (className === classCandidate) {
+          return `!${className}`
+        }
+        return className
       })
       r.walkDecls((d) => (d.important = true))
     })
@@ -514,7 +517,7 @@ function* resolveMatches(candidate, context) {
     matches = applyPrefix(matches, context)
 
     if (important) {
-      matches = applyImportant(matches, context)
+      matches = applyImportant(matches, classCandidate)
     }
 
     for (let variant of variants) {

tests/important-modifier.test.js

@@ -17,6 +17,22 @@ test('important modifier', () => {
       },
     ],
     corePlugins: { preflight: false },
+    plugins: [
+      function ({ theme, matchUtilities }) {
+        matchUtilities(
+          {
+            'custom-parent': (value) => {
+              return {
+                '.custom-child': {
+                  margin: value,
+                },
+              }
+            },
+          },
+          { values: theme('spacing') }
+        )
+      },
+    ],
   }
 
   let input = css`
@@ -57,6 +73,9 @@ test('important modifier', () => {
       .\!font-bold {
         font-weight: 700 !important;
       }
+      .\!custom-parent-5 .custom-child {
+        margin: 1.25rem !important;
+      }
       .hover\:\!text-center:hover {
         text-align: center !important;
       }