Add new diagnostics module

[wvhulle]

I am adding adding real-time diagnostics to Nu shell. To be able to show them, I need to add some features to Reedline. The goal is to give real-time feedback about partial Nushell prompts typed by the user.

In combination with nu-lint this could be helpful for new users.

• The diagnostics can have fixes that are exposed with a new "code action" Reedline menu that is opened with keybinding CTRL+.. Fixes are accepted with ENTER.
• Reedline would not depend on nu-lint but instead provide a mechanism for external "diagnostic providers" like nu-lint to send diagnostics to Reedline.

There is an example in the examples folder: cargo run --example diagnostics to see how the diagnostics currently look.

[wvhulle]

[fdncred]

I'm leaning against landing this right now. It's so much code and it seems like it would be super intrusive as you're typing. That may just be a preference though. I'm still considering it but I need some convincing.

[wvhulle]

I'm leaning against landing this right now. It's so much code and it seems like it would be super intrusive as you're typing. That may just be a preference though. I'm still considering it but I need some convincing.

I’ll configure it locally for my own shell and use it for a while on a daily basis to check if it is worth it and if it can be made less untrusive.

[wvhulle]

I made the diagnostics a bit less intrusive by keeping labels shorter, dimming colors, see this image:

[wvhulle]

@fdncred You can test on this branch (with recent Nu-lint binary):

cargo run --example lsp_diagnostics --features lsp_diagnostics

[wvhulle]

Fix menu with alt+f:

[blindFS]

@wvhulle Would you like to add the code action feature to nu-lsp and reuse it for reedline instead of implementing another lsp server? Or is there any technical issue that blocks this kind of communication?