Add basic mouse camera controls template for CharacterBody3D

[xam3adev]

What problem(s) does this PR solve?

The existing CharacterBody3D movement template provides basic movement, gravity, and jumping, but there is no template that includes basic mouse camera controls.

This PR adds a new CharacterBody3D template with basic mouse-look functionality.

The template allows users to:

Rotate the character horizontally using mouse movement.
Rotate the camera vertically using mouse movement.
Limit vertical camera rotation to prevent unnatural rotation.
Capture the mouse when the game starts.
Adjust mouse sensitivity.

This provides a more complete starting point for FPS/TPS-style 3D projects without requiring users to implement basic mouse camera controls themselves.

Additional information

The camera is expected to be a Camera3D child of the CharacterBody3D.

The template was written from scratch and tested in Godot to verify the movement, jumping, mouse-look, and mouse capture behavior.

[AThousandShips]

Did you test this? The template isn't correct it's not even a .gd file, also did you write the code, or did you just adjust AI authored code?

[xam3adev]

I tested it and I wrote it from the scratch

[AThousandShips]

The template isn't valid though, how did you test it? It won't be detected by the engine as it's not even a GDScript file