Generate CollisionLayers enum

[wjt]

In enums.gd we define enum CollisionLayers which has this comment:

## Please keep this in sync with Project Settings layer_names/2d_physics/

We could automate this! We could have a @tool script which rewrites this file on startup and in response to the ProjectSettings.settings_changed signal. (Or: rather than storing the enum in git, somehow dynamically generate it, create a GDScript resource from it and make it available at runtime.)

XP Summary (total 450)

Subdomain	Skill	XP
Engineering: Optimization & Performance	Problem Solving	150
Engineering: Optimization & Performance	Creativity & Design Thinking	150
Engineering: Optimization & Performance	Technical Literacy	150

[wjt]

I have to say that I'm not sure whether this is worth the complexity - but it might be an interesting project.

[wjt]

This is not really Optimization or Performance but it's also not really Gameplay or Systems Programming. We don't have a good label for "build system stuff."