[naga spv-out] Add f16 io polyfill

[cryvosh]

Connections
Extension of #7656

Description
Polyfills f16 I/O when storageInputOutput16 capability is missing on vulkan backend.
Emits these variables as f32 and inserts OpFConverts at loads/stores.
If capability is available, generated spirv is unaffected.

Testing
Added a snapshot test and some basic inspection of the spirv disassembly in spirv_capabilities.rs.
Generated spirv in snapshots look okay to me, but I haven't yet run any live correctness tests.

Squash or Rebase?
Squash

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.

[ErichDonGubler]

Mostly LGTM, but I noticed some things that seem like potential issues.

I also think the API could use some work, but let's iterate on that in follow-up PRs.

[jimblandy]

@cryvosh @ErichDonGubler What are the next steps here? I see questions from Erich that haven't been answered yet.

[cryvosh]

Hey sorry been busy, will address when I get a chance

[teoxoy]

Nice work!