Add language facet - feature flag

What does this MR do and why?

Some basic changes and updates to the sidebar in preparation for the language facet. The divider lines in the sidebar are now adhering to the design: #342651[Language_Facet_-_Checked.png]

set of MRs	status
Add language facet - feature flag	👈 this MR
Add language facet - add state and data	awaiting review
Add language facet - add components	awaiting review

Screenshots or screen recordings

How to set up and validate locally

1. Enable the language aggregation

   echo 'Feature.enable(:search_blobs_language_aggregation)' | rails c

2. Visit the search page http://gdk:3000/search
3. Scopes issues, and merge_requests will have correct dividers.

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 #342651 (closed)