refactor: move asset initialization from launchBattle to preload

[DayKev]

• Port of [Misc] Move asset initialization into preload instead of launchBattle pagefaultgames/pokerogue#6109

What are the changes the user will see?

N/A

Why am I making these changes?

Asset initialization should be done in the preload step and not after launching the game.

What are the changes from a developer perspective?

Moved asset initialization from BattleScene#launchBattle to #preload.
Removed utility function waitUntil in game-manager-utils.ts and replaced its use with vi.waitUntil.
Replaced firstTimeScene variable in GameManager with private resetScene method.
Simplified if/elseif in MoveHelper#selectTarget.

Checklist

• 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?
  • It could, but it's just a collection of small changes so w/e, might as well just port the whole thing at once.
• Have I provided a clear explanation of the changes?
• Have I tested the changes manually?