Count co-authors in on Contributors badge

:clipboard: **Description**

Add a boolean option on the Contributors badge to count co-authors in.
Currently, if I'm the sole contributor on a repo but someone co-authored a commit, the Contributors badge still displays 1.

Start by locating the Contributors badge entry point and its tests, then inspect how the current contributor count is obtained. Determine how the boolean option should be exposed and verify that enabling it counts co-authors while the default behavior remains unchanged.

Written by the indexing model from the issue text.