[wgsl-in, wgsl-out, glsl-in] WebGPU compliant dual source blending feature

[Wumpf]

Connections

• Fixes Converge dual-source blending with the WebGPU standard #6402
• Soft dependnencies:
  • Edit CHANGELOG a bit #7274
  • refactor: impl From<ImplementedEnableExtension> for EnableExtension #7275
  • docs(wgsl): use correct links to standard enable extensions #7276

Description

Makes the dual source implementation in wgpu WebGPU spec compliant.
This means changing the wgsl syntax & associated validation to match what's described here and here and here.
Furthermore, makes the dual source blending extension available when targeting WebGPU.

In order to reliably test wgsl output I incorrectly assumed that the way to go would be to add a glsl input to the snapshot tests (mostly to check for the enable directive being written out correctly), didn't realize that there's wgsl->wgsl snapshot testing as well, so I ended up implementing glsl index parsing as collateral😄

Testing
Added a slew of naga tests. Did manual testing in a pet project against native & WebGPU

Draft TODO

• [help wanted:] need to validate the the right enable directive is set but can't quite figure out how to access this in the right places
• [help wanted] [partially orthogonal]: While trying this out on my pet project I couldn't get it to fly in WebGPU because I was using naga_oil (with a hack, see also Support wgsl diagnostic directives. bevyengine/naga_oil#113 (comment)) where I have to rely on Module->write_wgsl which right now ignores all directives 😱 . Fixing that isn't entirely straight forward since Module is language agnostic so I have to detect the need for those
• a more vertical wgpu test checking that happy path works and that shaders needing wgsl require the device to have the correct feature set

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[ErichDonGubler]

I've done some significant reviewing today, and I intend to finish on my Monday. Looking mostly good so far, with most of my nits on diagnostics. 🙂

[ErichDonGubler]

Sigh, I haven't been able to finish this review because of some resource conflicts in other parts of my day job and personal life. I do intend to finish this soon, though.

[ErichDonGubler]

Approving, conditional on outstanding conversations being resolved. Since most conversations should be resolvable without code changes here directly, I want to unblock merging.

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_{I used Conventional Comments in this review! I hope they help with clarity and tone. 🙂}