Add experimental reference panel behind feature flag

Created by: mrnugget

This is the first, experimental, tech preview, proof-of-concept version of a better reference to get internal user feedback.

This fixes https://github.com/sourcegraph/sourcegraph/issues/30969

How to try this

• Check out the branch
• sg start enterprise-codeintel
• Auto-index a repository to get LSIF data
• Edit user settings
• Aet "experimentalFeatures": { "coolCodeIntel": true }
• Reload
• Hover over token & click
• New panel should open