Folder organization and enable github action

[WisdomToNorth]

• Copy the API headers to build/include, making it easier for API users
• Enable GitHub Actions on Windows, Mac, and Linux
• Temporarily disable unit tests combine_plines_test, cavc_get_extents, and combine_with_self_invariants which are currently failing

[jbuckmccready]

I think changes mostly makes sense but without cavc in the directory path it makes it less clear when including headers in c++ and the c api header shouldn't be in a src directory.

Probably should follow something like what is suggested here (note use of named directory in path for namespace/library clarity): https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1204r0.html#intro

[WisdomToNorth]

Thank you for the feedback. I agree with your points and have made the necessary adjustments to the commit. Please review the changes, thanks!