refactor: introduce THEMES env var

[mxschll]

Description

Allows dynamic inclusion of themes using THEMES=visual-refresh,one-theme npm run build. This will populate INCLUDED_THEMES which we then can use in dev pages. Introduces theme= url param.

Note: We can remove the visualRefresh=true param in as a next step.

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 97.52%. Comparing base (82d11d6) to head (07e5084).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4642   +/-   ##
=======================================
  Coverage   97.52%   97.52%           
=======================================
  Files         948      948           
  Lines       30363    30363           
  Branches    11077    11077           
=======================================
  Hits        29610    29610           
  Misses        706      706           
  Partials       47       47           

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