Mark code only used by #Preview macro as unused

[danwood]

Fixes issue #1046

This yields unused warnings (unless --retain-swift-ui-previews option) on any code only used by #Preview code.
The newer #Preview macro works quite differently than the old PreviewProvider, so the algorithm to detect it is a bit more involved. I've created some tests, and I've also exercised it on a big code base with lots of code that's only used by its previews, that Periphery wasn't picking up on.

[ky1ejs]

Thanks so much for handling this @danwood! And thanks all for this awesome software.

@ileitch would it be possible to merge and release this? Would love to have it. Just discovered some views that weren't caught as unused because of this issue.

[ileitch]

Thanks for taking a go at this, @danwood. I took a different approach in #1051 and #1056 that's a bit more generalized, and requires less specific handling of #Preview