Add Autoprefixer postcss Plugin

[coliff]

Fixes: #4157

Confirmed this is working as expected. The generated CSS in dist for user-select includes this now:

  -webkit-user-select: none;
  -moz-user-select: none;
  user-select: none;

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.54%. Comparing base (57716b0) to head (9c0ee27).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4165      +/-   ##
==========================================
+ Coverage   86.52%   86.54%   +0.02%     
==========================================
  Files         242      242              
  Lines       33071    33071              
  Branches     2031     2034       +3     
==========================================
+ Hits        28614    28621       +7     
- Misses       3466     3469       +3     
+ Partials      991      981      -10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.