Rename/alias "Run flycheck" command to something more obvious

I agree that flycheck isn't the best fit, but check seems really generic so I am not sure if its apparent what check necessarily means for people when they read it for the first time either? I guess Run cargo check would be better? (shame we can't have the command label be dynamic).

Alternatively we can start pushing the flycheck term so that people know what it means. After adding this I was thinking of maybe changing the name of checkOnSave configs to check, flycheck, etc since it's no longer only tied to saving

Originally posted by @Veykril in https://github.com/rust-lang/rust-analyzer/issues/13785#issuecomment-1357837161

I found the above comment while trying to find the name of this command, which does not seem to be very discoverable with cargo check or rust check in the VSCode command palette:

Start by locating the VSCode command registration for the current “Run flycheck” command and inspect how its label appears in the command palette. Rename or alias it so users can find it through cargo check or rust check, then verify the updated wording in the VSCode command palette.