[wgsl-in] external texture support

[jinleili]

Add support for texture_external.

WGSL spec: https://gpuweb.github.io/gpuweb/wgsl/#external-texture-type

Related issues:
#274
#1733

[ErichDonGubler]

In case it helps anyone, these sections of current spec. drafts for the Web platform seem the most relevant:

• Section 6.4 of the v1 WebGPU API spec. (GPUExternalTexture).
• Section 5.4.4 of the v1 WGSL spec. (External sampled texture types).