chore: Convert to monorepo

[ThangHuuVu]

Reasoning 💡

As discussed via #3650, this is the first step toward the final goal: Move all next-auth projects to a monorepo.

1. Convert this repo into monorepo with app and src using the chosen stack. Rename app and src into for example dev-app and core.

Checklist 🧢

Setup monorepo toolings

• Setup yarn workspaces
• Setup turborepo:
  • Installed turbo, added turbo.json.
  • Integrate with CI
• Setup changesets:
  • Installed @changeset/cli.
  • Remove semantic-release. Change version of next-auth to 4.1.2.
  • Integrate with CI

Existing commands & packages

• root's package.json:
  • eslint, typescript, prettier are kept in root
  • Keep version-pr
• core: is now packages/next-auth.
• dev-app: is now packages/dev-app.
• Documentation
• Ready to be merged

Some highlights:

• react and react-dom are set to nohoist
• npm run dev is now yarn turbo run dev. There's no need for npm run dev:setup
• copy:css and watch:css is moved to next-auth
• dev-app/tsconfig.json no longer needs path alias

Affected issues 🎟

Partially solves #3650