html: Add outline (#37098)

We were missing an outline definition for HTML flies, hence this PR adds
one for that

<img width="255" height="726" alt="image"
src="https://github.com/user-attachments/assets/ae59cb8d-6c69-4019-966a-d5baf744329d"
/>

Release Notes:

- N/A

extensions/html/languages/html/outline.scm

@@ -1 +1,5 @@
 (comment) @annotation
+
+(element
+  (start_tag
+    (tag_name) @name)) @item