Use @tag.doctype for HTML doctype highlights (#18024)

This PR updates the following extensions to use the `@tag.doctype`
selector for highlighting HTML doctypes:

- Astro
- Elixir (HEEx)
- HTML

Additionally, it also changes the base selector for HTML tags from
`@keyword` to `@tag`.

| Before | After |
|
-------------------------------------------------------------------------------------------------------------------------------------------------
|
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|
| <img width="308" alt="Screenshot 2024-09-18 at 2 04 41 PM"
src="https://github.com/user-attachments/assets/818d98ba-fce7-4683-b67f-61c86543831c">
| <img width="358" alt="Screenshot 2024-09-18 at 2 05 00 PM"
src="https://github.com/user-attachments/assets/5071db7c-e0bf-44df-8959-38275833833b">
|

Extracted this from https://github.com/zed-industries/zed/pull/16723.

Release Notes:

- N/A

---------

Co-authored-by: 狐狸 <134658521+Huliiiiii@users.noreply.github.com>

extensions/astro/languages/astro/highlights.scm

@@ -1,6 +1,6 @@
 (tag_name) @tag
 (erroneous_end_tag_name) @keyword
-(doctype) @constant
+(doctype) @tag.doctype
 (attribute_name) @property
 (attribute_value) @string
 (comment) @comment

extensions/elixir/languages/heex/highlights.scm

@@ -27,7 +27,7 @@
 "=" @operator
 
 ; HEEx inherits the DOCTYPE tag from HTML
-(doctype) @constant
+(doctype) @tag.doctype
 
 (comment) @comment
 

extensions/html/languages/html/highlights.scm

@@ -1,6 +1,6 @@
-(tag_name) @keyword
+(tag_name) @tag
 (erroneous_end_tag_name) @keyword
-(doctype) @constant
+(doctype) @tag.doctype
 (attribute_name) @property
 (attribute_value) @string
 (comment) @comment