elixir: Capture identifiers as @variable (#22579)

- Closes: https://github.com/zed-industries/zed/issues/19382

Before/After
<img width="329" alt="Screenshot 2025-01-02 at 13 08 46"
src="https://github.com/user-attachments/assets/ede36fd3-ed55-4436-912c-bb8b7ad9b0cd"
/><img width="329" alt="Screenshot 2025-01-02 at 13 08 18"
src="https://github.com/user-attachments/assets/eb784bdc-fd13-487d-b6ed-c960d8020d9b"
/>


Release Notes:

- N/A

extensions/elixir/languages/elixir/highlights.scm

@@ -67,6 +67,8 @@
   quoted_start: _ @string.special
   quoted_end: _ @string.special) @string.special
 
+(identifier) @variable
+
 (
   (identifier) @comment.unused
   (#match? @comment.unused "^_")