`bigquery_magics.is_registered` global boolean

[tswast]

To support googleapis/python-bigquery-dataframes#1231, we need a way to see if the magics have already been used/registered explicitly with %load_ext bigquery_magics.

Let's add a is_registered global to https://github.com/googleapis/python-bigquery-magics/blob/main/bigquery_magics/__init__.py that we can check if the magics are already registered from bigframes.