Support new xcstrings format

[keith]

New with Xcode 15 there is a new way to create strings files with a visual editor in Xcode. They are stored with all translations in a single json file with the .xcstrings extension. The result of processing them is that it outputs .strings files so the final app files looks just like they do today (they're always dumped in the xml format though). Strings files and stringsdict files now say "legacy" in the Xcode UI.

This issue is just for tracking support of processing this new file type