Icon generation without "dirty" repo files

Previously, VersionIcon wrote generated icons straight into the tracked --appIcon asset catalog. Every configuration switch (e.g. Debug ↔ Release) re-wrote those files, leaving the source assets "dirty" after almost any build and polluting git diffs.

This release fixes this behavior. This PR adds an opt-in “generated asset catalog” mode so VersionIcon can write configuration-specific app icons into a separate .xcassets output directory, avoiding modifications to tracked source icon assets.

If the new parameter --outputAssetCatalog is not specified, VersionIcon behaves the same as before.