Issue title: Latest HSL format (color level 4) not supported in color picker
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- VSCode Version:
- OS Version: MacOS 11.0.1

Steps to Reproduce:

1. Open a CSS file in VS Code.
2. Use an HSL color using the new HSL format i.e. `color: hsl(0 0% 100% / 20%);`

[CSS color level 4](https://www.w3.org/TR/css-color-4/#the-hsl-notation) has since dropped the need for commas, the "a" in HSLA, as well as other formatting changes. 

Does this issue occur when all extensions are disabled?: Yes

<img width="372" alt="No support for Color Level 4" src="https://user-images.githubusercontent.com/9301411/100298081-0b135f80-2f45-11eb-94a9-9a0eb89c96e0.png">


 Comments: 
 Comment 0: (Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:
- [\[themes\] Support for hue, saturation, lightness (HSL) color codes for theming (#72945)](https://www.github.com/microsoft/vscode/issues/72945) <!-- score: 0.53 -->
<!-- potential_duplicates_comment -->
 Comment 1: IMHO this feature is needed to be in compliance with modern browsers.
 Comment 2: <!-- 6d457af9-96bd-47a8-a0e8-ecf120dfffc1 -->
This feature request is now a candidate for our backlog. The community has 60 days to [upvote](https://github.com/microsoft/vscode/wiki/Issues-Triaging#up-voting-a-feature-request) the issue. If it receives 20 upvotes we will move it to our backlog. If not, we will close it. To learn more about how we handle feature requests, please see our [documentation](https://aka.ms/vscode-issue-lifecycle).

Happy Coding!
 Comment 3: For me personally, I'd like support for the `rgb(0 0 0 / 50%)` CSS Color Module Level 4 format. Changing color values in Chrome DevTools has seemed to favor this format over `rgba(0, 0, 0, .5)` for some time now, so I think Code ought to implement it. Hope this is the correct issue to upvote.
 Comment 4: Verify that you can use the comma less formats for `rgb` and `hla`, including alpha
e.g. `rgb(0 0 0 / 50%)`
and that the color decorator is shown
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