Refactor GDScript/C# script templates logic to be editor-only

Not a full refactor as it still goes through ScriptLanguage so it's hacky,
but at least it can now compile without this.

modules/mono/editor/script_templates/CharacterBody2D/basic_movement.cs

@@ -0,0 +1,41 @@
+// meta-description: Classic movement for gravity games (platformer, ...)
+
+using _BINDINGS_NAMESPACE_;
+using System;
+
+public partial class _CLASS_ : _BASE_
+{
+    public const float Speed = 300.0f;
+    public const float JumpVelocity = -400.0f;
+
+    // Get the gravity from the project settings to be synced with RigidDynamicBody nodes.
+    public float gravity = (float)ProjectSettings.GetSetting("physics/2d/default_gravity");
+
+    public override void _PhysicsProcess(float delta)
+    {
+        Vector2 velocity = Velocity;
+
+        // Add the gravity.
+        if (!IsOnFloor())
+            velocity.y += gravity * delta;
+
+        // Handle Jump.
+        if (Input.IsActionJustPressed("ui_accept") && IsOnFloor())
+            velocity.y = JumpVelocity;
+
+        // Get the input direction and handle the movement/deceleration.
+        // As good practice, you should replace UI actions with custom gameplay actions.
+        Vector2 direction = Input.GetVector("ui_left", "ui_right", "ui_up", "ui_down");
+        if (direction != Vector2.Zero)
+        {
+            velocity.x = direction.x * Speed;
+        }
+        else
+        {
+            velocity.x = Mathf.MoveToward(Velocity.x, 0, Speed);
+        }
+
+        Velocity = velocity;
+        MoveAndSlide();
+    }
+}