Slate v2 Workspace Root Entry Closure

Goal

Make the workspace packages source-runnable under Yarn PnP instead of relying on built dist/ output just to resolve package-name imports.

Completed

• added workspace root index.ts entries for:
  • packages/slate
  • packages/slate-history
  • packages/slate-react
  • packages/slate-dom
  • packages/slate-browser
• kept the root entries tiny:
  • export * from './src'

This mirrors the slate-hyperscript workspace fix and makes the package roots source-addressable in the workspace even when dist/ output is stale or absent.

Verification

• yarn lint:typescript
• yarn workspace slate-react run test
• yarn workspace slate-browser test