Allow overriding font style and weight via experimental.theme_overrides in settings (#9122)

Release Notes:

- Added support for overriding the current theme's syntax font styles
and weights in settings
([#9121](https://github.com/zed-industries/zed/issues/9121)).

| Before | After |
| ------ | ----- |
| ![Screenshot 2024-03-09 at 22 20
01@2x](https://github.com/zed-industries/zed/assets/38924/c693468d-1e04-45b4-b7c0-869e2a22a44c)
| ![Screenshot 2024-03-09 at 22 21
09@2x](https://github.com/zed-industries/zed/assets/38924/d8b09676-dd8b-46ac-8e9d-6cf2094a9c7e)
|