dev: add desktop.ini files to gitignore

[Bertie690]

What are the changes the user will see?

N/A

Why am I making these changes?

Damo noticed that a desktop.ini file was getting tracked by Git.
This is a Windows-specific file created to track explorer.exe info, and is re-created each time the folder is opened.

What are the changes from a developer perspective?

Added desktop.ini (in all subdirectories) to the gitignore.

Removed the gitignore entries for assets into a submodule-specific gitignore file.

Checklist

• The PR content is correctly formatted:
  • I'm using beta as my base branch
  • The current branch is not named beta, main or the name of another long-lived feature branch
  • I have provided a clear explanation of the changes within the PR description
  • The PR title matches the Conventional Commits format (as described in CONTRIBUTING.md)
• The PR is self-contained and cannot be split into smaller PRs
• There is no overlap with another open PR

Does this require any additions or changes to in-game assets? If so:

• I have created an associated PR on the assets repository
  • If so, include a link to the PR here: [Dev] Create gitignore file pokerogue-assets#66