[Refactor] Move playerFaints to arena data

[Fontbane]

What are the changes the user will see?

• None

Why am I making these changes?

• In [Ability][Move] Last Respects Refactor and Full Implementation #5200, playerFaints was tracked oddly. It's stored by the Arena class but not by ArenaData. This meant it wouldn't save. So instead, playerFaints was added straight to sessionSaveData, and the Arena constructor as well as BattleScene.newArena() required an extra parameter just to pass sessionSaveData.playerFaints.

What are the changes from a developer perspective?

• Puts player faint count in ArenaData class so it's saved and loaded from there instead of at the root of session data.
• Removes unnecessary parameter from Arena constructor and BattleScene.newArena()

Screenshots/Videos

How to test the changes?

Checklist

• I'm using beta as my base branch
• There is no overlap with another PR?
• The PR is self-contained and cannot be split into smaller PRs?
• Have I provided a clear explanation of the changes?
• Have I tested the changes manually?
• Are all unit tests still passing? (npm run test)
  • Have I created new automated tests (npm run create-test) or updated existing tests related to the PR's changes?
• Have I provided screenshots/videos of the changes (if applicable)?
  • Have I made sure that any UI change works for both UI themes (default and legacy)?

Are there any localization additions or changes? If so:

• Has a locales PR been created on the locales repo?
  • If so, please leave a link to it here:
• Has the translation team been contacted for proofreading/translation?

[Fontbane]

Had a better idea actually