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/indents.scm

@@ -0,0 +1,6 @@
+(start_tag ">" @end) @indent
+(self_closing_tag "/>" @end) @indent
+
+(element
+  (start_tag) @start
+  (end_tag)? @end) @indent