Allow making m2m relation of a table to itself

I am building a database, in which one of the tables has a many-to-many relationship to itself. As far as I can see, this is not (yet) possible using .m2m() in sqlite-utils. This may be a bit of a niche use case, so feel free to close this issue if you feel it would introduce too much complexity compared to the benefits.

Example: suppose I have a table of people, and I want to store the information that John and Mary have two children, Michael and Suzy. It would be neat if I could do something like this:

from sqlite_utils import Database

db = Database(memory=True)
db["people"].insert({"name": "John"}, pk="name").m2m(
    "people", [{"name": "Michael"}, {"name": "Suzy"}], m2m_table="parent_child", pk="name"
)
db["people"].insert({"name": "Mary"}, pk="name").m2m(
    "people", [{"name": "Michael"}, {"name": "Suzy"}], m2m_table="parent_child", pk="name"
)

But if I do that, the many-to-many table parent_child has only one column:

CREATE TABLE [parent_child] (
   [people_id] TEXT REFERENCES [people]([name]),
   PRIMARY KEY ([people_id], [people_id])
)

This could be solved by adding one or two keyword_arguments to .m2m(), e.g. .m2m(..., left_name=None, right_name=None) or .m2m(..., names=(None, None)).

Start by tracing the Python .m2m() implementation and the schema it creates for the provided self-referential people example. Determine how the two sides of the relationship can receive distinct columns, then verify that both parent-child calls create the intended links in SQLite without collapsing the foreign keys.