Improve the CI by checking each package and feature separately.

[xStrom]

As was discussed in Zulip we've had hidden breakages lately. One major source of these issues has been Cargo's workspace dependency feature unification. So if some package in a workspace enables a feature, all other packages may incorrectly depend on that feature without specifying the requirement.

Our CI already split --lib & --bins from other targets for that reason, however the Vello repo has its examples as separate packages, and also just has more packages in general. Thus a more robust CI is needed.

Because we have a lot of different repos in Linebender, I strive to make changes in a widely applicable way. That means avoiding specifying package names if I can help it. Luckily cargo-hack has a much better --workspace implementation that doesn't do feature unification and instead runs the commands on each workspace member separately.

With the introduction of cargo-hack we now also have access to its feature matrix options. I went with --each-feature and --optional-deps to run checks with every feature separately. (In the future we can also consider adding a less frequent job with --feature-powerset --depth 10 or something like that to really test the combinations.)

I also converted the wasm32 job to be more like the main job and that already caught a bunch of issues.

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• Hid the CPU shaders behind the wgpu feature, as right now that is just the reality.
• Fixed the full feature of vello_encoding.
• Fixed the wgpu-profiler feature of with_winit when targeting wasm32.
• Added missing js feature to getrandom dependency.

[DJMcNab]

Thanks - it's good to get these sorted