Add support for swift source files

:boom:

Incomplete, it adds the files but I think we need to create a module.

- [ ] Support generating frameworks instead of static libraries (this should be done in a different issue/PR)
- [x] Add support for swift file extension
- [ ] Create bridging headers

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- [ ] Decide on the best way to design this as backwards compatible (vague, I know – swizzlr)