[Swift] Package manager support

[mackoj]

Hi,

Can you please consider providing an official ANTLR Swift runtime suitable for use from Swift Package Manager ? Like this one https://github.com/Flight-School/SwiftAntlr4 which is non-official but working and not maintained.

Thanks,

[flying-turnip]

Afaik this could be done similarly to the way the PHP runtime has been separated into it's own repo. I am under the same requirements of using a swift package, and decoupling the swift runtime files from this repo would essentially make that happen, while requiring very little effort to do so.