feat(output): Implement incremental save functionality for packs (fixes #512)

[Stoupy51]

Fixes #512

A new incremental option (default to false) can be given to the output plugin.
either OutputOptions or in beet.yml:

meta:
  output:
    incremental: true

When enabled:

• Files no longer in the pack are deleted individually instead of wiping the whole directory
• Possible empty directories left over are pruned bottom-up
• Files already on disk and unchanged are skipped
• Only new files (or modified) are written

File watchers will now be in pain each build.
On my tests, I get faster build time, idk if you want to make it True by default so I put at False.

[Stoupy51]

Damn, didn't thought copilot review would start automatically on another repo beside mines

[Copilot]

Pull request overview

This PR adds an incremental output mode to the output contrib plugin to avoid wiping and rewriting entire pack directories on every build, addressing performance issues for large projects (issue #512).

Changes:

• Add incremental: bool to OutputOptions (default False).
• Implement incremental_save() to delete removed files, prune empty directories, and skip writing unchanged files.
• Update output() to use incremental saving for non-zipped packs when enabled.

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