[EuiMention] - Component to mention users or hashtags

[elizabetdev]

Component idea

EUI should provide an easy way for consumers to mention users or hashtags in EuiMarkdownEditor.

• Consumers can pick what is the character to trigger a list of suggestions (e.g the character "@").
• Once a suggestion is picked, it renders a EuiLink (character + string) with the possibility of having a EuiTooltip.

Spec doc

Initially, all the discussions should happen in the following doc: https://docs.google.com/document/d/1nky8AS2a2NZqEcxlpec0gmm4ydMvx4Nx2NjxpL1xViQ/edit?usp=sharing.

Design

The source of truth for the design lives in Figma.

Figma prototype

You can find the prototype here.

Related issue

#4926