Introduce new Colour Select Widget

What does this MR do and why?

Implements a new Colour Select Widget.

The new widget was suggested in !72779 as part of allowing epics to have an assigned colour, and it was then suggested to be moved out into its own MR.

Screenshots or screen recordings

Taken from the original mockup	Example
	simplescreenrecorder-2022-06-07_20.27.23

How to set up and validate locally

This MR does not mount the component yet. It will be use in follow up MRs to implement the component into the "New Epic" page and the "View Epic" sidebar.

Epic colours are currently behind the epic_color_highlight feature flag.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #7641 (closed)

To Dos

• Update usages of color_item
• Pass in allowEdit prop to color_select_root
• Move component into vue_shared/components
• Rename to color_select_dropdown
• Remove issuableType from color_select_root and references to it
• Remove isDropdownVariantStandalone
• Include SuggestedColors namespace for the color names