refactor: Rewrite Pokemon generation

[innerthunder]

What are the changes the user will see?

N/A

Why am I making these changes?

The constructors for Pokemon and its subclasses are a mess. This is also a prerequisite for #1394.

What are the changes from a developer perspective?

• The Pokemon constructor's optional parameters have been consolidated into a PokemonOptions interface. It also accepts a Pokemon in place of the options interface to create shallow copies of Pokemon. Subclasses PlayerPokemon and EnemyPokemon have also been given similar treatment.
• Enemy overrides for shiny variant, IVs, nature, etc. are now all applied in the EnemyPokemon constructor.
• PokemonData now implements PokemonOptions, meaning it can also be directly plugged into Pokemon constructors.
• With the above changes, the dataSource constructor parameter has been made obsolete, hence it not being included in PokemonOptions.

Screenshots/Videos

How to test the changes?

Affected overrides may require playtesting. Unit tests are TBD

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)?