Add contextual music rarity

[CI09]

Brief Description of What This PR Does

Adds rarity constant and math for it

Related Issues

Closes #5552

Progress Checklist

Note: before starting this checklist the PR should be marked as non-draft.

• PR author has checked that this PR works as intended and doesn't
  break existing features:
  https://wiki.revolutionarygamesstudio.com/wiki/Testing_Checklist
  (this is important as to not waste the time of Thrive team
  members reviewing this PR)
• Initial code review passed (this and further items should not be checked by the PR author)
• Functionality is confirmed working by another person (see above checklist link)
• Final code review is passed and code conforms to the
  styleguide.

Before merging all CI jobs should finish on this PR without errors, if
there are automatically detected style issues they should be fixed by
the PR author. Merging must follow our
styleguide.

[revolutionary-bot]

We are currently in feature freeze until the next release.
If your PR is not just a simple fix, then it may take until the release to get reviewed and merged.

[dligr]

I have tested this to work in game. Code changes make sense to me too.

[hhyyrylainen]

I improved the memory handling and code clarity here. I also added a TODO comment about fixing playing the same track over and over with this new code: #5615

I'll merge this now and assume my changes didn't break the actual functionality here.