Add color-picker capabilities

[krassowski]

Elevator Pitch

LSP specifies a colour support capability which can be implemented as a textDocument/documentColor request. Quite a few language servers (CSS/R) support it well already.

Motivation

This is very useful for CSS, but also for plot tuning in data science area (right colours make the visualisations much better!).

Design Ideas

The easylogic/codemirror-colorpicker makes it easy to implement the colour picker in the CodeMirror editor (which is what we already use). It can look like this:

This can be implemented as a new CodeMirror adapter feature, see: /packages/jupyterlab-lsp/src/adapters/codemirror/features