Use pnpm and Nx to manage the monorepo

[nickgros]

TODO: Set up CI for v6
TODO: Test publishing and ensure release scripts work

Reasons for making this change

Fixes #3965

• Use pnpm for package management
• Improves developer experience by allowing us to reference workspace package dependencies that are replaced at publish-time, removing the need to release new versions just for the sake of upgrading peer dependencies ()
• Improves developer experience by being faster than npm
• Fully use Nx for monorepo management (drop Lerna aliases, which just used Nx under the hood)

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature