Add TryGetExtension methods AL & ALContext matching those BuildTools generates for OpenGL (et al)

[Perksey]

Because the OpenAL bindings are handwritten (and the first set of bindings written for Silk.NET) they do not match our other bindings. One of the most notable differences is the lack of the TryGetExtension method in our OpenAL bindings, which is present in all of our other bindings.

This issue is to add a TryGetExtension method to the AL and ALContext classes. This method is simple (see here) and should be able to be just copied and pasted cleanly over.

Community contributions welcome.

[Perksey]

This has been fixed in #598.