Support brotli compression

Please add brotli compression support to CSS API.

Currently, the CSS API uses gzip compression, but the CSS size will be large if using a lot of fonts or some fonts with a large number of characters, and worse if using multiple weights.
Most browsers already support brotli, use brotli encoding will significantly save network traffic.

Some font CSS test[^1]:
| Font | Origin | Gzip | Brotli |
| ------------------- | ---------: | ---------: | --------: |
| [Noto Sans TC] | `482368` | `134474` | `20741` |
| [IBM Plex Sans JP] | `464852` | `122655` | `19626` |
| [Noto Color Emoji] | `8085` | `2118` | `1541` |
| [Roboto] | `17720` | `983` | `785` |

[^1]: All sizes in bytes.
Using gzip and brotli CLI tools with default compression level.

[Noto Sans TC]: https://fonts.googleapis.com/css2?family=Noto+Sans+TC:wght@300;400;500;700&display=swap
[IBM Plex Sans JP]: https://fonts.googleapis.com/css2?family=IBM+Plex+Sans+JP:wght@300;400;500;700&display=swap
[Noto Color Emoji]: https://fonts.googleapis.com/css2?family=Noto+Color+Emoji&display=swap
[Roboto]: https://fonts.googleapis.com/css2?family=Roboto:ital,wght@0,300;0,400;0,500;0,700;1,300;1,400;1,500;1,700&display=swap