Adds the support for marking entites and fetching them in queries
View more examples here
```rust
enum Players { Red, Blue, }
struct CurrentPlayer;
struct Player(u32);
fn setup(
mut commands: Commands,
mut markers: MarkerMut
let blue = commands.spawn(Player(7)).id();
markers[Players::Blue] = blue;
**current = blue;
}
fn getredplayer(mut query: Query<&mut Player>, markers: Marker
fn getcurrentplayer(mut query: Query<&mut Player>, current: Marker
at your option.