Fixed that query.iter().len() is 0 on the first frame.

[BritishWerewolf]

With the changes in this PR, the marker structs will be registered in the PostStartup schedule (in addition to the original Update) allowing for any schedule that follows this to access the query.

One of the things I've noticed is when using this within a query, the query isn't populated immediately.
I think this is due to the Changed filter.

Consider the following example, that now works:

fn player_movement(mut query: Query<(&mut Transform), With<Enum!(app::Tag::Player)>>) {
    let mut transform = query.single_mut();
    // Do some mutations here...
}

This is not possible in the original code, and in fact, if we were to just change the body to println!("{}", query.iter_mut().len()) then we'd see the console print the value 0, before printing a sea of 1.

Currently we have to implement the code in the following way:

fn player_movement(mut query: Query<(&mut Transform), With<Enum!(app::Tag::Player)>>) {
    if query.is_empty() {
        return;
    }

    let mut transform = query.single_mut();
    // Do some mutations here...
}