Migrate packages to MetaMask npm org

[rekmarks]

After the MVP, we should either migrate the packages in this repo to the MetaMask npm org (as their own packages or as part of existing ones), or delete them:

• @mm-snaps/controllers: Refactor @mm-snaps/controllers #12
• @mm-snaps/post-message-stream
  • Merge into the regular post-message-stream.
• @mm-snaps/workers
  • Make compatible with above WebWorkerController changes.
• @mm-snaps/rpc-methods
  • We can't do anything with this until after the controllers have been migrated. It may serve as a useful point of reference for the methods that the controllers will need to provide.
  • It should probably continue to be its own package.
• @mm-snaps/types
  • Determine whether this package makes sense after everything else is migrated or if we should just get rid of it.

[rekmarks]

This was done.