feat: implement F16 support in shaders

[FL33TW00D]

Connections

• Hard dependency on WGSL: Add a skeleton for enable-extensions/enable …; that reports nice errors #6424.
• Resolves WGSL: Support the enable directive #5476.
• Resolves [meta] f16 support #4384.

Checklist for merge-ability

• Test multiple redundant directives (I think there may be a cleaner way to do this: https://github.com/gfx-rs/wgpu/pull/5701/files#diff-9ef835ffc5f7d7313243db5bfa371d3115b82c0705d4f1df75c98dfe07d927a1R2175)
• MSL out
• HLSL out
• SPV out
• Arbitrary for F16 on half-rs: feature: Arbitrary starkat99/half-rs#111. Currently we are using a git dep on my fork which is causing CI to fail.
• hexf_parse f16 support: https://github.com/lifthrasiir/hexf. This one won't be fun. Did some work on it here: lifthrasiir/hexf@master...FL33TW00D:hexf:feature/f16 . Deferred to Implement Parsing of Hexadecimal F16 #7046
• We should make sure that we are properly validating As casts of matrices, because that code currently assumes that all matrices have the same kind (Naga's typechecker ignores scalar kind when casting matrices #6441).
• Validate out f16 inside push constants.
• Continue tests for f16 layout.
• wgsl-out needs to enable f16
• Fix spirv output of matrix casts
• Aliases for f16 types still don't seem to work
• Split out out-of-scope commits

Additional context

F16 is available in >=SM6.2: https://github.com/microsoft/DirectXShaderCompiler/wiki/16-Bit-Scalar-Types

[FL33TW00D]

I've marked this as ready for review, as the wgpu specific logic is implemented and would be great to start iterating on it.

2 main blockers are:

1. Merging of the PR on half-rs.
2. hexf-parse improved implementation.

[FL33TW00D]

Dogfooding works! Fixed a few bugs in 30e12b5. Still waiting on upstream.

[teoxoy]

One thing I'm concerned is that the polyfills we have in some of the backends might not support f16. It would be worth adding more tests covering the built-in functions and operations that support f16. Execution tests would be great as well, they live in tests/tests/shader.

[cwfitzgerald]

Restored ownership of the commit, sorry about that :)

[raphamorim]

man it's happening, let's go! ❤️

@cwfitzgerald @ErichDonGubler please lemme know if i can help somehow