Add basic common support

[Vipitis]

part of #4 (mainly in preparation for adding Buffers A-C, audio shaders and loading form API eventually)

The common tab (sometimes called renderpass by me due to API key-naming) is essentially a shared "library" of functions and variables across the different buffers (including default/image). Information can be found here

The implementation simply puts it between the combability code and the shader_code which might not be correct in all cases
So more stuff to check before this is ready:

• test wgsl
• test macros and builtins (according to the blog post)
• better examples

[Korijn]

LGTM!