chore: Adds theme optimisation and default theme layer

[pan-kot]

Changes

The PR introduces default CSS layer wrapping the base theme. This is a preparation for the follow-up change to generate override themes as .css files (see prototype: cloudscape-design/components#4416), in addition to the current build-time theming approach. The override themes will use a separate layer with a higher priority to ensure it is applied on top of the base theme.

Additionally, the PR includes several output optimisations (AWSUI-16016):

1. Single and multi transformers produce identical rules (expected) that can be merged together;
2. The media queries of the adjacent selectors are merged;
3. Adjacent selectors having the same media and definitions are merged in a single comma-separated selector.

Testing

1. New unit tests
2. Existing tests including snapshot tests
3. Manual verification in the components repository
4. Screenshot tests in the internal pipeline
5. Dry run to live

In the components repository, I ran the project build to see how the changes affect the generated base-component styles, where the theme declarations live:

1. old-styles.scoped.css - 279Kb / 33.2Kb gzipped
2. new-styles.scoped.css - 213K / 29.6Kb gzipped

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.57%. Comparing base (f183e40) to head (941270e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   97.52%   97.57%   +0.05%     
==========================================
  Files          48       48              
  Lines        2621     2637      +16     
  Branches      508      549      +41     
==========================================
+ Hits         2556     2573      +17     
+ Misses         65       64       -1     

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[gethinwebster]

looks good!