This shader attempts to replicate Godot's default lighting, meant to be used as a starting point for creating a custom lighting shader.
To be more specific, it replicates the look of the Godot 4.1.1 standard material with the Lambert diffuse mode, and the Schlick-GXX specular mode.
The shader is essentially a very barebones version of the internal Godot shader.
Here's quick comparison of the shader and the standard material with various roughness settings, for both metallic and dielectric materials:
Happy shader coding!