Move convert_* functions into gpuconvert.rs

[eerii]

Centralizes all conversion functions between wgpu/webgpu bindings in one module, components/script/dom/gpuconvert.rs.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes partially fix Create convert mechanisem for WGPU <=> webgpu #31104
• These changes do not require tests because they don't introduce any new functionality

[sagudev]

Great stuff, thanks for all the work.

This is only part of the issue (to put all convert files into one place), but I still haven't designed mechanism (macro+traits) for two way conversion.

[eerii]

Oh that makes a ton of sense, two way conversion would be way nicer! Thanks for reviewing and changing the description to match :D