Extract HTML support into an extension (#10130)

This PR extracts HTML support into an extension and removes the built-in
HTML support from Zed.

Release Notes:

- Removed built-in support for HTML, in favor of making it available as
an extension. The HTML extension will be suggested for download when you
open a `.html`, `.htm`, or `.shtml` file.

extensions/html/languages/html/injections.scm

@@ -0,0 +1,7 @@
+(script_element
+  (raw_text) @content
+  (#set! "language" "javascript"))
+
+(style_element
+  (raw_text) @content
+  (#set! "language" "css"))