feat: 🎸 add rust fly check command

[IWANABETHATGUY]

1. rust cargo check after each save is pretty costly, especially when the project became very large.
2. flyCheck is very useful in such a scenario, after running this command rust analyzer will on demand trigger cargo check and push the diagnostics into neovim lsp client

[IWANABETHATGUY]

https://user-images.githubusercontent.com/17974631/236387035-ff2f43eb-8e99-4ece-a587-5677407ac924.mp4

[IWANABETHATGUY]

related code in rust-analyzer vscode extension https://github.com/rust-lang/rust-analyzer/blob/master/editors/code/src/lsp_ext.ts#L45-L47

[pgherveou]

Looks pretty cool, do you need to disable checkOnSave when you want to use this?

[IWANABETHATGUY]

In theory, yes. Because if you enable checkOnSave, save will trigger a fly check already. This is only useful when cargo check cost pretty long time.

[MunifTanjim]

I've pulled this in https://github.com/MunifTanjim/rust-tools.nvim/tree/patched

[pgherveou]

In theory, yes. Because if you enable checkOnSave, save will trigger a fly check already. This is only useful when cargo check cost pretty long time.

So basically you only need to make sure you have this checkOnSave turned off to benefit from that feature. Might be worth adding a note about it in the doc so that people knows about it