Would allow for app authors to make async calls within their UI function. From what I can see, https://github.com/posit-dev/py-shiny/blob/3e587abacacc0cc8482d7edaa9f5b05aed732e4e/shiny/_app.py#L305 is the only place it is executed and is already within an async function. We could wrap it in async at the very beginning, avoiding repeated _is async_ checks.