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.

• [x] 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)
• [x] Initial code review passed (this and further items should not be checked by the PR author)
• [x] 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.