[9.x] Vendor publish flag that restricts to only existing files

[intrepidws]

I often find myself overriding a specific view file for a vendor package and then losing track of whether the package eventually updated that file or not. While I could simply run php artisan vendor:publish --tag=filament-config --force, that publishes ALL of the view files when I am only interested in files that I have created manually. Since many packages have dozens, if not hundreds, of view files - this can be burdensome.

This PR introduces a new --existing flag that instructs the vendor:publish command to only publish and overwrite files that already exist.

I did consider naming the flag --force-only-existing, or some variation thereof and would be open to any change you felt would be more appropriate.