Add a Display filter to allow folks to filter the visibility of QuickPost

[aurooba]

This PR closes #4, and adds a new prop to the QuickPostButton component called visibility, as well as wrapping that component in WithFilters to allow filtering with addFilter by other people.

The updated README includes an example on how to change when QuickPost is visible using the filter.

import { addFilter } from "@wordpress/hooks";
import { useSelect } from "@wordpress/data";
import { store as coreStore } from "@wordpress/core-data";

function onlyAdmins(FilteredComponent) {
	const adminUser = () => {
		const { isAdmin } = useSelect((select) => {
			const { canUser } = select(coreStore);
			// check if current user can create users, because only admins can do that
			const isAdmin = canUser("create", "users"); // returns undefined, true, or false
			return {
				isAdmin: isAdmin,
			};
		});
		return isAdmin;
	};
	return (props) => (
		<>
			<FilteredComponent {...props} visibility={adminUser()} />
		</>
	);
}

addFilter(
	"QuickPost.Display",
	"my-plugin/only-admins-quickpost",
	onlyAdmins
);

[aurooba]

Changed the filter to be called QuickPost.Display so that it's easy to add more filters later if needed in the same format.