Detect if we're not in a WebGPU capable browser with an Instance that targets the BROWSER_WEBGPU backend.

[Elabajaba]

Is your feature request related to a problem? Please describe.
When you create an Instance that targets the BROWSER_WEBGPU backend, but not the GL backend, Instance::create_surface panics if you're using a browser that doesn't support WebGPU.

Describe the solution you'd like
Some way of knowing that we're not in a WebGPU capable browser with just an Instance

Describe alternatives you've considered
Instance::enumerate_adapters() doesn't work because it's always None for WebGPU backends.

[Wumpf]

It feels like we need to iterate on this whole store again a bit. Hit very related issues there:

• even if a browser advertises webgpu support it may still fail to create an adapter, see workarounds for this in If WebGPU fails, re-try adapter creation with WebGL emilk/egui#3895
• Allow checking type of wgpu::Instance's context #5142

(edit: linked wrong issue/pr at first)

Not yet sure exactly how to deal with all of this, but I think wgpu should expose the necessary tools to work with this in a sane(r) way.
The way enumerate_adapters is a wgpu-core only function is quite confusing, wondering if we should just bite the bullet and make it async in order to merge these paths again