WGSL implement texture intrinsics except gather and sampler-less (#5123)

This commit implements all of the texture intrinsics for WGSL except "Gather" and sampler-less.
They will be implemented in a separate PR.

A few things to note:

 - texture sampling functions are available only for the fragment shader stage; not for compute
 - WGSL doesn't have any functions similar to CalculateLevelOfDetail or CalculateLevelOfDetailUnclamped.
 - WGSL doesn't have a function overlaoding for textureSample with "clamp" or "status" arguments.
 - WGSL doesn't support Load operation with offset for texture_multisampled_XX and texture_storage_XX.
 - WGSL supports only four types of depth textures: 2D, 2D_array, cube and cube_array.
 - WGSL doesn't support "offset" variants for cube and cube_array.

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