Make Pattern Library put extra routes in default group if group is not set

[lyzadanger]

Per this PR: hypothesis/client#3738

This manifested as a type-linting problem, but I think it overlays an ergonomic one.

It's a bit of a pain for authors to have to provide a group, and that that group needs to conform to a somewhat arbitrary set of groups. A couple of possibilities:

• Make group optional (or even disallowed) for "extra" routes provided by external applications. By default, put these routes into a group that is defined by the pattern library (not externally).
• Perhaps stop restricting group names for external applications (i.e. allow external applications to use whatever "groups" they want).

[lyzadanger]

Now part of #450