Add function to distinguish custom-added and built-in atlases

[tsalo]

@effigies proposed using the following in sMRIPrep where it would be helpful to distinguish between custom and built-in atlases:

import zipfile
import templateflow

templates = [
    tpl.name.removeprefix('tpl-')
    for tpl in zipfile.Path(templateflow.conf.load_data('templateflow-skel.zip')).iterdir()
]

It would be great to have a function that does this (e.g., templateflow.api.get_builtin_templates()).