gopls reports Go `const` declarations as `variable` with the `readonly`
modifier. Add a Go-specific semantic token rule to map this to the
`constant` theme style, matching VS Code's behavior.
<img width="465" height="32" alt="image"
src="https://github.com/user-attachments/assets/e0bc8a60-2275-4f81-9185-8b86c51d3d08"
/>
Closes#50642
Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
Release Notes:
- N/A *or* Added/Fixed/Improved ...
Nianyu Shen
0edc702262