10998: Add number representation assists r=Veykril a=errx Reimplemented assists from this PR https://github.com/rust-analyzer/rust-analyzer/pull/3683 with current APIs.   I've decided not to add options about size of the groups so behaviour is similar to clippy's. Minimal number length is also taken from clippy. Co-authored-by: Oleg Matrokhin <matrokhin@gmail.com>