vkd3d-shader/hlsl: Implement inverse trigonometry.
Implements asin, acos, atan, and atan2.
Also includes some tests in a new test file.
One possible problem here is that I'm not sure how to test what Microsoft's atan and atan2 outputs are in boundary cases like atan2(1, 0). I've made the test suites adhere with the calculator program I've been using (Qalculate, which I assume is using libc's atan2).