This PR introduces `@tldraw/tldraw/tldraw.css`, an exported CSS file
that replaces the `editor.css` and `ui.css` that were previously copied
from the editor and ui packages. Instead, these files are combined into
the `tldraw.css` file, simplifying the import story when using
`@tldraw/tldraw`.
### Change Type
- [x] `major` — Breaking change
### Release Notes
- [tldraw] Removes `editor.css` and `ui.css` exports, replaces with
`tldraw.css`
4dfc59e5cb