Add :host rule to @theme layer

Fixes #15799

Adds a `:host` selector for the `@theme` layer. This is necessary for the `@theme` layer to work correctly in shadow DOM.

Also updates the snapshots for the tests that were affected by this change.

Author: hugo-vrijswijk

Diff for: packages/tailwindcss/src/index.ts

@@ -473,7 +473,7 @@ async function parseCss(
       // Keep a reference to the first `@theme` rule to update with the full
       // theme later, and delete any other `@theme` rules.
       if (!firstThemeRule && !(themeOptions & ThemeOptions.REFERENCE)) {
-        firstThemeRule = styleRule(':root', node.nodes)
+        firstThemeRule = styleRule(':root, :host', node.nodes)
         replaceWith([firstThemeRule])
       } else {
         replaceWith([])