Add pnpm -F autocomplete support

[kamnakis]

Sanity checks

• [X] I have searched github.com/withfig/fig/issues and there are no duplicates of my feature

Feature Details

Description:

It would be very nice to be able to get autocomplete for all the available workspaces in a pnpm monorepo (pnpm filtering).

[mschrage]

Moving this to the autocomplete repo. The pnpm spec is open source and anyone can contribute new generators.