Add github CI for verifying builds and tests

[G4Vi]

This adds free automated test building and running to the project on Windows and Linux hosts using github actions.

If this is added, whenever a push or pull request is made the tests will be built and ran and it'll spit out a report like: https://github.com/G4Vi/dr_libs/runs/4743920379?check_suite_focus=true

[ktmf01]

You could also use the testvectors here: https://github.com/ietf-wg-cellar/flac-test-files/tree/main/subset

These FLAC files are specially crafted to test decoders, and are already on github.

[G4Vi]

You could also use the testvectors here: https://github.com/ietf-wg-cellar/flac-test-files/tree/main/subset

These FLAC files are specially crafted to test decoders, and are already on github.

That does look like a good source of test files! If mackron is interested in the PR I'll switch to some of them, but I'm going to hold off doing more work on this until confirmation.