refactor: Rework Trainer generation

[innerthunder]

What are the changes the user will see?

TBD

Why am I making these changes?

The current Trainer generation system has several glaring issues that limit its usefulness:

• There is virtually no documentation for the data structures and methods involved.
• There's no separation between the TrainerConfig data structure and the API for constructing it.
• The party template system is very rigid, and its role in Pokemon generation isn't immediately clear through the API.
• Much of TrainerConfig is built around the assumption that Trainers can only be either male or female.
• The Trainer class has too many responsibilities. As a Phaser sprite subclass, it should at most include methods for the trainer's visual effects, not the ~800 loc dedicated to config access, Party Pokemon generation, and other miscellaneous things.

What are the changes from a developer perspective?

WIP

Screenshots/Videos

How to test the changes?

WIP

Checklist

• ⚠️ If this is a PR for main (such as a hotfix), has the game version been updated (pnpm update-version:patch / pnpm update-version:minor?
• Otherwise: 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? (pnpm test:silent)
  • Have I created new automated tests (pnpm test:create) 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 (dark and light)?

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:
• Have I added the Localization tag to this PR?

If there are no locale changes:

• Have I made sure not to commit any changes to the locale repo on this branch?