[Misc] Clean up the Battle class a bit

[DayKev]

What are the changes the user will see?

N/A

Why am I making these changes?

General code cleanup.

What are the changes from a developer perspective?

Various instances of public var; this.var = 0; turned into public var = 0;
An instance of !! was replaced with ?? false
A couple of comments were made into tsdoc comments.

How to test the changes?

npm run test

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 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)?
• Have I tested the changes (manually)?
  • Are all unit tests still passing? (npm run test)
• [ ] Are the changes visual?
  • [ ] Have I provided screenshots/videos of the changes?

[torranx]

while you're at it, can you please change the integer types to number?

[DayKev]

Good point.