Support setting font feature values (#11898)

Now (on `macOS` and `Windows`) we can set font feature value:
```rust
  "buffer_font_features": {
    "cv01": true,
    "cv03": 3,
    "cv09": 1,
    "VSAH": 7,
    "VSAJ": 8
  }
```

And one can still use `"cv01": true`.



https://github.com/zed-industries/zed/assets/14981363/3e3fcf4f-abdb-4d9e-a0a6-71dc24a515c2




Release Notes:

- Added font feature values, now you can set font features like `"cv01":
7`.

---------

Co-authored-by: Mikayla <mikayla@zed.dev>