[Refactor] Clean up `StatStageChangePhase` and `PokemonPhase`

[DayKev]

What are the changes the user will see?

N/A

Why am I making these changes?

Because it was there.

What are the changes from a developer perspective?

The const end = () => { ... }; inside of the start() method was moved into its own class method. An unused method aggregateStatStageChanges() was removed. player was renamed to isPlayer in PokemonPhase for clarity.

How to test the changes?

Use moves that alter stat stages.

Checklist

• [x] I'm using beta as my base branch
• [x] There is no overlap with another PR?
• [x] The PR is self-contained and cannot be split into smaller PRs?
• [x] Have I provided a clear explanation of the changes?
• ~[ ] Have I considered writing automated tests for the issue?~
• ~[ ] If I have text, did I make it translatable and add a key in the English locale file(s)?~
• [x] Have I tested the changes (manually)?
  • [x] Are all unit tests still passing? (npm run test)
• ~[ ] Are the changes visual?~
  • ~[ ] Have I provided screenshots/videos of the changes?~