Support setting a primary key as a foreign key reference, for m2m tables

[simonw]

Fixed this problem but found you cannot set a primary key as a foreign key reference, which prevents you from using this plugin to create a m2m table with foreign key references. I'll fix that in a separate issue.

Originally posted by @simonw in #51 (comment)

[simonw]

This currently isn't possible because of this code:

datasette-edit-schema/datasette_edit_schema/__init__.py

Lines 436 to 449 in 92cff82

	column_foreign_keys = [
	{
	"name": column["name"],
	"foreign_key": foreign_keys_by_column.get(column["name"])[0]
	if foreign_keys_by_column.get(column["name"])
	else None,
	"suggestions": [],
	"options": integer_primary_keys
	if column["type"] is int
	else string_primary_keys,
	}
	for column in columns
	if not column["is_pk"]
	]