WebGPU spec compliant dual source blending

[Wumpf]

Connections

• Fixes Converge dual-source blending with the WebGPU standard #6402

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.

Testing
Added a slew of naga tests. Did manual testing in a pet project against native & WebGPU (see also draft todo for ongoing problems)

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.

[Wumpf]

oops, didn't push this to my fork