Migrate to a monorepo

[Gugustinette]

Discussed with @sxzz , we should move to an all-in-one architecture :

• Configure current package.json for monorepo, use pnpm workspaces,...
• Create a packages/ directory with :
  • tsdown-cli/ being the current create-tsdown cli, renamed
    • Definitely move the migrate command to the cli
  • create-tsdown/ reworked to just be an alias to npx tsdown-cli create
• Move current create-tsdown templates in a dedicated tsdown-templates repo
  • (optional) Transfer the repo to the Rolldown organization