Allow fully custom config for check and format commands

[chriskrycho]

Is your feature request related to a problem? Please describe.

I am doing some work on the rust-lang/rust repo, which has some unique needs for using rust-analyzer successfully. That repo does not work with normal rust-analyzer.

Describe the solution you'd like

The recommended workflow uses a JSON config for the VS Code integration. I don’t think this needs to fully support that, exactly, though it certainly could, but it would be nice if it were possible to supply the full rust-analyzer option set—either in the UI directly, or by loading a config file like that.

Describe alternatives you've considered

I can just use VS Code, or I can just work with ./x check on the terminal directly. 😓

Additional context

I know this is weird. 😂 The number of people who work on the Rust project itself and use Nova is probably literally just me.