Handlers should run in a custom monad that doesn't expose stuff that people shouldn't use

[michaelpj]

Today our handlers have access to all of LspM, which lets them:

• Access the VFS and the configuration
  • This is incorrect - plugin handlers should be accessing the snapshots of these that we have in the shake graph
• Send arbitrary messages
  • This makes things harder to test, we can't just look at the handler return value to see the main thing that the handler "does"

We should probably run in a custom monad that is a wrapper around LspM that restricts some stuff, and maybe also change our handler types to mostly return their results.

[awjchen]

I'd like to work on this!