feat: Add reference tokens to theming API in Core

[jkuelz]

Description

This won't be merged until the prerequisites have been merged, but this is in preparation for release. It makes the reference token API available to use for customers and in our demos.

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 97.12%. Comparing base (b5728e6) to head (83b916f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4087   +/-   ##
=======================================
  Coverage   97.12%   97.12%           
=======================================
  Files         864      864           
  Lines       25327    25327           
  Branches     9140     9141    +1     
=======================================
  Hits        24599    24599           
+ Misses        722      681   -41     
- Partials        6       47   +41     

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