This PR adds a sneaky action / kbd (Alt-F) for the fill fill style.
Similar to the white action, this is not really documented and may be
removed in the future. But it's cool!!
### Change Type
- [x] `sdk` — Changes the tldraw SDK
- [x] `feature` — New feature
### Test Plan
1. Press Alt-F.
### Release Notes
- Secretly adds a fill-fill style (Alt-F)
a307939822
