[naga wgsl-in wgsl-out] WGSL support for texture_external texture type

[jamienicol]

Connections
Part of #4386

Description
Make wgsl-in correctly parse texture_external texture declarations, and allow such textures to be used in textureDimensions(), textureSampleBaseClampToEdge(), and textureLoad() function calls. In IR these are represented by the ImageClass::External image class, which is a 2D, non-multisampled, non-mipmapped, float-sampled image.

Adds a new Capability TEXTURE_EXTERNAL and ensure validation rejects shaders containing external textures if this capability flag is not set. This capability is enabled for validation by wgpu devices which support the TEXTURE_EXTERNAL feature (currently only when using the noop backend), and by the Naga CLI when validating-only or when outputting WGSL.

The WGSL backend can of course emit ImageClass::External images directly as texture_external textures. Other backends are, for now, unimplemented.

Lastly, we add a snapshot test covering all the valid uses of a texture_external texture. These are:

• As a global variable declaration
• As an argument to the built-in functions textureDimensions(), textureSampleBaseClampToEdge(), and textureLoad()
• As an argument to user-defined function declarations and calls.

We keep these in their own test so that we can control which targets to run them against (currently WGSL and IR). When external textures are supported by all Naga backends we can, if so inclined, integrate these with existing texture tests.

Testing
Snapshot test

Squash or Rebase?
Either. But if squashing make sure the commit message is tidy

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

I don't think this by itself deserves a changelog entry - when we add at least one non-wgsl backend that would be more noteworthy