Add `Spawner2D` node - Githubissues

Resolves godotengine/godot-proposals#3651.

Spawn nodes from scenes for scripts automatically per time step
Specify both spawn rate and time step
Can start spawning with a delay, useful for implementing spawn waves.
Can limit the number of nodes spawned in order to automatically stop the spawner.
The spawned nodes can be instantiated locally or globally, so that the spawner's position can be freely animated without affecting the transform of nodes that were already spawned.

Note that I'm not using Timer nodes for this at all, the timer logic is custom and allows for better integration and control.

Spawner3D could be implemented similarly.

Note that not everything below need to be implemented, this is rather a list of ideas.

[x] Add node_spawned signal (when spawn() is called, either manually or per timer) and finished signal (when limit > 0).
[ ] Optional spawn sequences (add Position2D to specify additional spawn points to override the default spawn at origin).
- Note the sequential part. The spawner would traverse each Position2D in succession deterministically. This should be useful for things like explosion sequences.
[ ] Spawn shapes: point (current), rectangle, circle, polygon etc.
- This asks for adding randomization features to the class, but could be useful for effects rather than gameplay logic, yet this could be achieved with Particles2D anyways.
[ ] Automatic lifetime management
- Not sure about this one, there are numerous ways to delete objects, and the condition to free nodes may be complex, but could mostly be useful for particle-like spawning of scenes. The video above makes coins manage lifetime within the coin scene instead.

goostengine / goost