Merge #5885

Merge #5885

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>

![demo](https://user-images.githubusercontent.com/36276403/91238334-77fc3b00-e745-11ea-836b-2822015ece98.gif)

</details>

Co-authored-by: Veetaha <veetaha2@gmail.com>