Add web command to request usb access

[mjbvz]

This experimental command triggers a request for usb devices. After the user selects these, extensions under the extension host can access them using navigator.usb.getDevices(...)

[bpasero]

I think contrib is a bit overkill for this, if we think as a platform web USB should be supported, I would make this an official extension command somewhere where other extension commands contribute from core.

[alexdima]

👍 Nice! works for me with await navigator.usb.getDevices(). I've tested in the extension host web worker context.