[css] API/configuration to extend the CSS language server

[aeschli]

microsoft/vscode-css-languageservice#44 wants to extend the CSS language server with a formatter.

[octref]

@aeschli Would you want to use prettier which has support for css/scss/less for this one? Vetur is using prettier and people like it.

[aeschli]

This is just about adding API so that other products can add it to the language server.
For VSCode this should come from an extension.

[octref]

Do you mean having extension contributing to the css-language-feature extension? I don't feel that complexity is needed. The real blocker is #41882 which doesn't handle multiple formatter as good as we do for completion items / hover. IMO a better solution should be making it possible for multiple registrations of formatters, and making the formatters selectable.

[aeschli]

It's about new language server API. See microsoft/vscode-css-languageservice#44 as the motivation for this request.