Run task to completion in `wasmtime run --invoke 'foo()'`

[dicej]

Currently, wasmtime run's invoke_component function uses Func::call_async to invoke the function. However, that will only run the guest task until it produces a result, then print that result and exit. Ideally, it would use Func::call_concurrent instead, which returns a TaskExit which can be used to await the completion of the task, allowing any post-return operations to finish.