Update EntityManager to use a read-only wrapper over entity groups

[hak33m16]

TODO: Look into if there's an interface to implement toArray() and forEach() just so we can be consistent with other iterable objects

Makes it a lot nicer to work with the EntityManager for queries now

const group: EntityGroup = entityManager.queryComponents(Position)
for (const [id, entity] of group) {
  // dostuffs
}

becomes:

const group: EntityGroup = entityManager.queryComponents(Position)
for (const entity of group) {
  // dostuffs
}

Also, because the query doesn't directly pass back the map anymore, this group is basically a read-only copy. The only remaining gotcha now is that if users store this group, they could be surprised that it changes over time as entities are modified since the .get() method is really accessing a group reference underneath

To combat that issue, users can create their own copies of the contents of the query/group:

const myCopy = Array.from(entityManager.queryComponents(Position))

(since Array.from() will work on anything that implements Iterable)

Also adds in a .toArray() and .forEach() function