Add LSP flychecks

[onbjerg]

Describe the feature

LSPs usually have support for flychecks, which are commands that are run on e.g. save. For rust-analyzer, this would be either cargo check or cargo clippy, which add their own diagnostics on top of rust-analyzer's own by sending diagnostics over stdout in a JSON format.

We should add flychecks to the LSP too with the following features:

• Multiple flychecks should be allowed, which are invoked on save.
• If we detect that forge lint is available, we should default the flycheck to forge lint.

Blocked by #417

Additional context

No response