Theme types cannot have characters such as . or /

### Tested versions

Godot v4.4.dev unknown - Windows 10.0.19045 - OpenGL 3 (Compatibility) - NVIDIA GeForce RTX 4050 Laptop GPU (NVIDIA; 32.0.15.6070) - 13th Gen Intel(R) Core(TM) i5-13500HX (20 Threads)

### System information

Godot v4.4.dev unknown - Windows 10.0.19045 - OpenGL 3 (Compatibility) - NVIDIA GeForce RTX 4050 Laptop GPU (NVIDIA; 32.0.15.6070) - 13th Gen Intel(R) Core(TM) i5-13500HX (20 Threads)

### Issue description

Theme types cannot contain characters such as periods (.) or slashes (/)

I couldn't find information about this on the documentation but I could just be blind. I think it would be cleaner if you could use names with periods as if they were sub-classes of the main type.

### Steps to reproduce

1. Create a theme
2. Set a name such as Button.Test
3. See it does not add

### Minimal reproduction project (MRP)

N/A