2D movement overview

**Your Godot version:**
v4.3.stable.official [77dcf97d8]

**Issue description:**
`res://scripts/projectile.gd:13 - Parse Error: Identifier "velocity" not declared in the current scope.`
2D movement overview doesn't work on Area2D nodes as specified in the documentation

`We'll use [CharacterBody2D](https://docs.godotengine.org/en/stable/classes/class_characterbody2d.html#class-characterbody2d) for these examples, but the principles will apply to other node types (Area2D, RigidBody2D) as well.`

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/tutorials/2d/2d_movement.html#introduction