Simplify nav hovers

[janbrasna]

👷 Running into some WebKit bug ignoring thickness on direct flex element. (May need a tweak in actual markup, or just living with it…)

One-line summary

Simplifying hover underlines on main menu, dropdowns and footer links: Moving from borders or fills on pseudo elements to native text decoration for better resilience.

Significant changes and points to review

Accomplishing the same effect while being less fragile.
(Real typographic underline makes descenders more legible by avoiding them instead of crossing.)
Changing to relative distance to adapt better to type scaling.

🚧 WIP
(Status: MVP-level, missing some QA and tree shaking, cleanup and tracking down more leftovers no longer needed. TBD next steps about WebKit if no solution deems practical.)

Issue / Bugzilla link

Fixes #15850

Testing

Ready to test, nb: difference in thickness in main nav between Gecko/Blink vs. WebKit…

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.55%. Comparing base (9db97b8) to head (f847aee).
Report is 145 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15855      +/-   ##
==========================================
+ Coverage   79.22%   79.55%   +0.33%     
==========================================
  Files         159      160       +1     
  Lines        8287     8436     +149     
==========================================
+ Hits         6565     6711     +146     
- Misses       1722     1725       +3     

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