Add Sonoma+ wallpaper setter

[clay-golem]

This should set the wallpaper across all spaces and displays for Sonoma and later.
It modifies a plist, so I’ve added logic to create a backup of the original file.
I’m not sure how this worked before Sonoma, and I don’t have a Mac to test it, so I’m simply checking the OS version and falling back to the old setter in the plugin if needed.
There’s also a defaults read 'com.apple.wallpaper' 'SonomaFirstRunMigrationPerformed' flag, which I believe is used by the wallpaper agent to migrate older wallpapers after upgrading to Sonoma. But again, I have no way to test this—so just in case, I treat its absence or a value not equal to 1 as if the OS is older than Sonoma.

[clay-golem]

It'd probably be good to add some simple type checking. Something like:

[[ "$(file -b "$1")" =~ (JPEG|PNG|HEIF|BMP) ]]

Though I don't know the whole range of acceptable types.

[rgcr]

Thanks for the PR, we went with a simpler approach, after setting the wallpaper via osascript, we just clear the Spaces and Displays entries from the plist and restart WallpaperAgent.
This forces macOS to apply the wallpaper to all spaces without needing a template file.
Check the changes in commit 49036f0