5885: Make inlay hints colors more configurable r=matklad a=Veetaha **[BREAKING CHANGE]** Tackles https://github.com/rust-analyzer/rust-analyzer/issues/5337#issuecomment-680018601 and generally related to #5337. Added `foreground/background` color configurations with optional more specific overrides `foreground.(type|parameter|chaining)Hints`. One problem I see is that the config keys are long and don't fit into the on-hover hints in the `settings.json` file entirely... <details> <summary>Demo</summary>  </details> Co-authored-by: Veetaha <veetaha2@gmail.com>