Adding in new --runtime web-extension feature

[Pauan]

Since it's specific for web extensions, the runtime is much smaller (it doesn't need Node support, or instantiateStreaming fallbacks, or UMD).

I didn't see any tests for the other runtime options, so I didn't add any.

[koute]

Thanks for the PR!

It'd be nice to have a test purely to make sure we don't break this functionality by a mistake in the future. (e.g. we could probably add a testcase like the build_and_run_native_wasm in integration-tests which would compile a test crate and run it with Node.js using a simple shim similar in concept to src/test_runner.js which would inject a fake getURL and eval the thing)

But looks good to me otherwise! Thanks!