chore: Add themeable design tokens

[at-susie]

Description

Introduces new public, themeable design tokens for the Badge component's border and text colors, fixes a token rename for expandable section heading font size, and adds a borderWidthBadge token.

Note: All added or modified tokens default to their original values, so there is no visual change to existing components.

Related links, issue #, if available: n/a

How has this been tested?

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 97.43%. Comparing base (97ff38c) to head (b2c25c6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4560   +/-   ##
=======================================
  Coverage   97.43%   97.43%           
=======================================
  Files         941      941           
  Lines       29725    29725           
  Branches    10804    10803    -1     
=======================================
  Hits        28962    28962           
- Misses        716      756   +40     
+ Partials       47        7   -40     

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