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

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.