chore: Reduce duplicated styles on button component

[avinashbot]

Description

Created to "unblock" #3253. We're at the edge of a couple of the bundle size checks, and adding a new button variant pushed it over the edge. Taking a look at the CSS, I realized we were looping through and generating a chunk of CSS for each button variant, but a lot of it wasn't variant-dependent, so it didn't really belong in that for loop.

Just like that! A lot of pointless CSS removed (though with no particular user impact since I imagine it could easily be gzipped away)

  lib/components/internal/plugins/index.js
  Size limit: 14 kB
  Size:       13.79 kB with all dependencies and minified
  
  lib/components/internal/widget-exports.js
- Size limit: 800 kB
- Size:       796.35 kB with all dependencies and minified
+ Size limit: 750 kB
+ Size:       745 kB with all dependencies and minified

How has this been tested?

Will run it in my pipeline to make sure the selector order changes didn't mess anything up visually. But it seems reasonable enough to have a reviewer assigned in the meantime.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.44%. Comparing base (f8a00dd) to head (b03fdec).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3254    +/-   ##
========================================
  Coverage   96.44%   96.44%            
========================================
  Files         791      791            
  Lines       22568    22568            
  Branches     7385     7793   +408     
========================================
  Hits        21765    21765            
+ Misses        796      750    -46     
- Partials        7       53    +46     

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[just-boris]

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