Use double comments in shader template light functions

[tetrapod00]

This PR adds a second layer of commenting to the light() function of CanvasItem and Spatial shader templates, so if you select and uncomment the light() function, you have a valid shader.

Current behavior:
If you create a new Spatial or CanvasItem shader, the template comes with a commented out light() function:

But if you select it and uncomment it with ctrl+k, you have a shader with a compile error, since the template only uses a single level of commenting:

This PR:
Tested on the CI artifact:
Shader template:

Select and uncomment the light function, and you have a valid shader:

[akien-mga]

Thanks!