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

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.