Change `wasmtime serve` to apply `-S common` by default

People keep running into problems with `wasmtime serve` because by default it only exposes `wasi:proxy`, nothing else. I think we should change the behavior here to imply `-S common` to make default toolchains and in particular the normal Preview1 adapter work.

One could see it as a downside that `wasmtime serve` would be a bit less specifically about HTTP, but I can't really come up with compelling arguments for why that'd be a problem.