4700: Add top level keywords completion r=matklad a=mcrakhman
This fixes the following issue: https://github.com/rust-analyzer/rust-analyzer/issues/4566.
Also added simple logic which filters the keywords which can be used with unsafe on the top level.
Co-authored-by: Mikhail Rakhmanov <rakhmanov.m@gmail.com>
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>