Feature/wgsl intrinsic texture gather (shader-slang#5141)

This PR implements the texture gather functions for WGSL.

The pattern was very similar to how Metal was implemented.
Before copy and paste from the Metal implementation, I had to
clean up the Metal implementation to make it more readable
and maintainable.

Gather functions are available only for 2D and 3D textures.
Their `array` and `depth` variants may or may not be supported depending on the target.
`static_assert` ensures that Gather functions are available only for 2D and 3D textures.

Removed incorrect use of "$p" argument for targeting GLSL.