Factor out Teleporter, Cinematic, CollectibleThread scene switching logic

[wjt]

We currently have at least 3 reusable entities in the game which take the player to another scene:

Node	Behaviour	spawn_point_path	Transition
Cinematic	Plays dialogue, then (optionally) switches to another scene	Supported but without a drop-down menu in the inspector	Can't be configured
Teleporter	When the player enters the Area2D, teleports to another scene, or within the same scene	Supported, with drop-down menu in inspector	Configurable
CollectibleItem	Optionally plays dialogue, then optionally switches to another scene	Not supported	Not configurable

(We also have some more bespoke components that switch scene, such as the eternal loom, but I'm ignoring that because you can never refactor away all the code in a project!)

In the same vein as #1346, it is annoying that they have different features. For example, right now I want to adjust the tutorial quest so that collecting the final thread takes you to the starting island of Fray's End rather than to the Eternal Loom, but CollectibleItem doesn't let you do that.

I think we should have some generic node that has all the configuration of Teleporter. Then, all three of the nodes above could use it.

[wjt]

I'm making this a base class because it's by far the easiest way to expose all these properties on each of the three components, and I don't fancy going through every scene to split Cinematic and CollectibleItem into two nodes (and have the learning team update their course materials).