[shader-ast] Missing matrix types

[sylee957]

The matrix types are missing,
especially implementing the rectangular ones could be involved due to the shape checking, but could be useful.

mat2x2
mat2x3
mat2x4
mat3x2
mat3x3
mat3x4
mat4x2
mat4x3
mat4x4

[postspectacular]

They're still missing since I've got limited bandwidth and not had any need for either of them so far (apart from mat2x3 which I thought would be there...) Happy to accept PRs, if you feel so inclined! 😉