Specify a different `provider`

[RemiDesgrange]

The JSPM generator API

The parameter cannot be overriden in the importmap.toml (IIRC the marshmallow here: https://github.com/dropseed/django-importmap/blob/master/importmap/core.py#L26)

The only solution I can see is to have a custom ImportMap class, but currently we can't customize that via a const in settings.py. (Or I'm missing something)