Add issue popover component

What does this MR do and why?

This adds the frontend for the new issue popovers. This won't enable the feature yet because this requires new data attributes in the Markdown HTML that will be added in !88000 (merged)

This frontend MR needs to be merged and deployed first so that the FE code is ready to handle both old and new Markdown. Without this MR, the new Markdown won't have tooltips or popovers.

#25607 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Cherry pick c500fa14 locally
2. Add an issue reference in a note or issuable description. Markdown preview also works.
3. Hover over the issue reference

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.