feat(extension,kernel): Move test functionality into vat

[rekmarks]

Requires #57 #28 #29

The CommandMethod enum more or less represents our provisional kernel API, currently exposed via globalThis.kernel() in the background service worker. This provisional API is currently implemented in offscreen.ts, and consists mostly of functionality that is only relevant for testing. We still want to keep this testing functionality around, but it should be implemented as user code running in a vat, as opposed to directly implemented in an arbitrary place in the application.

[rekmarks]

We deleted all of the methods that were purely for testing, and will rely on booting different vats to test things from hereon out.