Draft: Prototype context menu in color picker

Add a context menu to the OKLab ("OKHSL") color picker, allowing the user to easily copy CSS color values to the clipboard.

Partial progress on inbox#7862.

Current limitations

• Only in the OKHSL picker at the moment;
• Only the color wheel is right-clickable;
• Only two color formats supported at the moment (hex RGB and oklch — part of CSS4);
• No icons in the menu (preference not respected);
• Appearance not finalized.

This prototype would greatly benefit from the following MRs being merged first: !6150, !6260 (merged), !6290 (merged).