[IMP] base: add _is_internal method to res.users

No method was readily available to know if a user is `internal` (has
group `base.group_user`), which was inconsistent with other base groups.

_is_internal is now used in the codebase where it is clear that
`.has_group('base.group_user')` is called on a single record.

Part-of: odoo/odoo#85703