Integrate retrie with HLS or hie-bios

[pepeiborra]

Standalone retrie cannot run the renamer since it doesn't know how to set up the GHC session, as discussed in #10. To address this, I propose integrating retrie as a command in HLS, reusing the ability for plugins to declare new commands added in haskell/haskell-language-server#1999. This will give retrie access not only to the renamer and typechecker, but also to the hiedb.