Migrate to pnpm and modify directory structure

[pmalacho-mit]

lerna has made installing more difficult than necessary (currently we're frozen at v3 which seems to require pkg-config to be installed on the user's machine, which I believe is no longer included with the latest version of MacOS).

We should leverage pnpm's workspace capability to accomplish the same thing lerna was doing.

Consider the following structure:

NOTE: I propose we make use of the $ prefix to indicate that certain folders are part of the extension framework and not extensions themselves (inside the extensions directory).

• extensions
  • $common
  • $documentation
  • $testing
  • $srcripts
  • simple_example
  • selfieSegmentation
  • ...etc
• scratch-packages
  • scratch-vm
  • scratch-gui