Provide a way to toggle rust-analyzer on and off

[ntc2]

Or maybe this already exists and I just can't find it?

Anyway, my use case is that rustic mode is often becoming very laggy or freezing when I'm fixing conflicts during rebasing, I'm guessing because a bunch of files change at once and need to be reanalyzed. In these cases, I usually don't need rust analyzer anyway, because I'm just fixing a bunch of obvious small conflicts. All I usually care about in these cases is the auto-format-on-save functionality, and the ability to run rustic-cargo-check.