Remake the 2d/platformer demo from scratch

This Godot project is a complete rewrite of the official Platformer 2D demo. This is part of a proposal to improve and harmonize the official demo projects. We want to:

- Teach good programming practices with Godot.
- Bring demos to a higher quality standard.
- Unify the code in the demos.

See the [open issue](godotengine#390) for more information.

This new demo showcases features from the original, and works with Godot 3.2.

Co-authored-by: Johnny Goss <me@johnnygoss.dev>