5682: Add an option to disable diagnostics r=matklad a=popzxc
As far as I know, currently it's not possible to disable a selected type of diagnostics provided by `rust-analyzer`.
This causes an inconvenient situation with a false-positive warnings: you either have to disable all the diagnostics, or you have to ignore these warnings.
There are some open issues related to this problem, e.g.: https://github.com/rust-analyzer/rust-analyzer/issues/5412, https://github.com/rust-analyzer/rust-analyzer/issues/5502
This PR attempts to make it possible to selectively disable some diagnostics on per-project basis.
Co-authored-by: Igor Aleksanov <popzxc@yandex.ru>
