Support iOS 15/macOS 12

[liamnichols]

• Closes: Support macOS 12/iOS 15/etc by using String(localized:) instead of LocalizedStringResource #30

Bit of a WIP, but the initial draft that will allow use of xcstrings-tool on iOS 15/macOS 12.

I still need to add documentation back to the generated code, but its mostly there so can be tested out if you need it urgently.

For a strings table called Localizable with a key called foo, you can use as follows:

String(localizable: .foo) // iOS 15+ via String.init(localized:defaultValue:table:locale:bundle:)
String(localized: .localizable.foo) // iOS 16+ via LocalizedStringResource

[liamnichols]

Will merge this as it is complete. I want to refactor the generator code at some point, but at least we have a good base now.

I'm going to add the ability to alter the locale using the iOS 15 API in a followup Pull Request.