Check all cfgs

[lylythechosenone]

I often develop both sides of a feature (flag on and off) at once, and it's very annoying to have to change my settings.json just to get intellisense on the default-disabled features. I'd like it if rust-analyzer just checked both sides of it separately (as a setting ofc). That would also result in better errors. It could be as simple as checking with both --all-features and --no-default-features.