chore: Move style utils to internal radio button component

[jperals]

Description

Small follow-up from #4021.

Move style utilities that are specific to the (internal) radio button component also into the radio button directory, to keep dependencies unidirectional: radio group depends on radio button and not the other way around.

How has this been tested?

• Existing tests
• Ran in my pipeline as part of feat: Add radio button component to Core #3860, then split it into this separate PR

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 (c02fc0d) to head (93a3546).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4049   +/-   ##
=======================================
  Coverage   97.12%   97.12%           
=======================================
  Files         864      864           
  Lines       25325    25325           
  Branches     9137     9138    +1     
=======================================
  Hits        24597    24597           
+ Misses        722      681   -41     
- Partials        6       47   +41     

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