Make sure the fill HTML dependency comes for the ride with page_fillable()

[cpsievert]

Closes #860

[wch]

This function is being called for its side effect, modifying the object in place. I think in all other instances in the code base, we call it for its return value.

The function definition has a question of how it should work:

py-shiny/shiny/ui/fill/_fill.py

Lines 33 to 40 in ddfd762

	@add_example()
	# TODO-future-API; These methods mutate their input values. Should they return a new object instead? Or not return at all (like `LIST.sort()`)?
	def as_fillable_container(
	tag: TagT,
	) -> TagT:
	tag_prepend_class(tag, FILL_CONTAINER_CLASS)
	tag.append(fill_dependency())
	return tag

I think that for clarity and consistency, that function should only be used for its return value, and shouldn't alter the object in place. I'll make a follow-up PR that changes that function, and also alters the usage here to match.