refactor: Move WidthType enum into BevelEdgesModifier class

- Included the `WidthType` enum directly within the `BevelEdgesModifier`
class, as it is logically tied to its behavior.
- Removed the separate class file for `WidthType` to simplify the
structure and maintain better cohesion.
- Improved organization and reduced unnecessary complexity by
encapsulating the enum where it's most relevant.

Author: ArtifactForms