Improve ruby highlighting (#7829)

Release Notes:

- Improved syntax-highlighting of identifiers in Ruby.

Before:

![screenshot-2024-02-15-14 40
19@2x](https://github.com/zed-industries/zed/assets/1185253/29fca0eb-7c0c-4aee-9f31-a8a3c6680cb9)

After:

![screenshot-2024-02-15-14 40
56@2x](https://github.com/zed-industries/zed/assets/1185253/2ce0e0c9-8689-4ff8-9f40-2ea5f6ccc2f6)