Add support for Apple String Catalogs

[mangerlahn]

Apple introduced a new format for storing localizations in 2023. A string catalog (.xcstrings) is a single file containing localizations, including plurals, for multiple languages. The format is based on JSON. I was not able to find a spec online, but there is a session explaining how to work with these files. It may also be that Apple considers this format private, as they advertise using .xliff files when exporting the localized strings. What do you think?

Brought over from Weblate: WeblateOrg/weblate#10848

[mangerlahn]

Apple also updated their identifiers for plurals in .xliff files. Plus, they added syntax for conditionals to provide additional strings under certain conditions like device type etc.. These changes to xliff files apply only when String Catalog files are used within Xcode. I believe both types of syntax can occur within one xliff file. More at 23:30 in the above mentioned session.

[nijel]

It's always annoying to implement support for a format that is not documented. We can definitely look for existing examples (like https://github.com/liamnichols/xcstrings-tool-demo/blob/main/DogTracker/Localizable.xcstrings), but that doesn't have to cover all the features and might be fragile. If the format is desired for interoperability, having a JSON schema for it would be the best approach.

The good thing on using the XLIFF ids is that the XLIFF file will just work (similar to #4614). The only drawback is that the plural forms will be shown as individual forms instead of as a plural forms of a single string.

[nijel]

Quoting https://forums.developer.apple.com/forums/thread/732120?answerId=756602022#756602022:

As for the specifics of the JSON format itself, we have not published documentation because it is primarily intended to be read/written by Xcode. If you do end up writing your own parser for this format, please be aware that Xcode reserves the right to change it.

So they apparently consider the format private and will not be helpful. Still, from a Weblate perspective, it might be useful to support it.