Add interpolation option for varying in WGSL

[XanthosXanthopoulos]

In the current version, when preprocessing a WGSL shader the interpolation options are ignored making it impossible to use non f32 based types which require @interpolate(flat). With the proposed changes both interpolation type and sampling can be specified in a similar fashion as in GLSL shaders.

Reference: https://www.w3.org/TR/WGSL/#interpolation

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[sebavan]

LGTM, thanks !!! Let s wait on @Popov72 to be fully safe. :-)