This PR refactored the turbo renderer architecture to support multiple block layout types.
- New base class `BlockLayoutPainter` and `BlockLayoutProvider` are introduced for writing extendable per-block layout querying and painting logic.
- Paragraph-specific lines are all moved into dedicated classes (`ParagraphLayoutProvider` and `ParagraphLayoutPainter`) under the `/variants/paragraph` dir.
- The `renderer-utils.ts` doesn't contain paragraph-specific logic now.
- The `text-utils.ts` is also now scoped for paragraph only.
- Worker messages are now strongly typed.
Upcoming PR should further implement the block registration system using extension API. The `variants` dir could still exist, since there will be similar rendering logic that can be reused among block types (i.e., between paragraph block and list block).
