What problem does this feature solve?

Currently only official plugins can respect the useConfigFiles option when creating a project when defined in a preset (or manual mode). Right now 3rd party plugins have the option of making use of extendPackage or using render to write out the file manually. This would mostly be for 3rd party plugins to use extendPackage and not worry about where the config ends up.

What does the proposed API look like?

Adding a method like addConfigTransform(key, transform) to GeneratorAPI that adds transforms to an object on the Generator instance of the GeneratorAPI. Then when extractConfigFiles is eventually called the transforms can be resolved to Object.assign({}, configTransforms, this.configTransforms). It'd be simple enough to prevent adding transforms for something like the vue config (or maybe all of the defaults).

Would this be viable?