Add helper function to coerce ui functions into express context managers

[cpsievert]

Right now we've blessed a subset of UI functions as "express-able" context managers (i.e., they work with with).

Seems it'd be worth having a helper function to help transform UI functions into express-able ones? For example:

from shiny import ui
from shiny.express import as_express_ui

page_navbar = as_express_ui(ui.page_navbar)

[cpsievert]

Oh and it looks like we already an internal way to do this:

py-shiny/shiny/express/_recall_context.py

Lines 71 to 78 in a5de5fa

	def wrap_recall_context_manager(
	fn: Callable[P, R]
	) -> Callable[P, RecallContextManager[R]]:
	@functools.wraps(fn)
	def wrapped_fn(*args: P.args, **kwargs: P.kwargs) -> RecallContextManager[R]:
	return RecallContextManager(fn, args=args, kwargs=kwargs)
	return wrapped_fn