Clangd uses the "readonly" token modifier instead of "constant".
Therefore, "readonly" should be mapped to highlight constant variables
as constants.

Before:
<img width="706" height="48" alt="before"
src="https://github.com/user-attachments/assets/8cc2a310-7825-490b-b868-58ea231612fa"
/>

After:
<img width="762" height="46" alt="after"
src="https://github.com/user-attachments/assets/e271d4cd-cc59-45f9-a8b4-2885857915db"
/>

- [x] Code Reviewed
- [x] Manual QA

Release Notes:

- Added clangd readonly token modifier to semantic token rules to
highlight constant variables as constant.

Co-authored-by: ozacod <ozacod@users.noreply.github.com>