refactor: CSS layer name and new tests

[pan-kot]

I changed the default layer name from "cloudscape-base-theme" to "awsui-base-theme" to be more generic - this matches the prefixes we use for design tokens and custom data attributes.

Additionally, I added new unit tests that ensure that runtime themes have priority over build time themes because the former do not include the CSS layer.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.58%. Comparing base (43861b7) to head (96948ed).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #173   +/-   ##
=======================================
  Coverage   97.57%   97.58%           
=======================================
  Files          48       48           
  Lines        2641     2651   +10     
  Branches      549      545    -4     
=======================================
+ Hits         2577     2587   +10     
  Misses         64       64           

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