[3.14] gh-135676: Add a summary of source characters (GH-138194) (GH-139781)

(cherry picked from commit 59a6f9d8c52af40ccd31e5dca848f71808c24b06)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Blaise Pabon <blaise@gmail.com>
Co-authored-by: Micha Albert <info@micha.zone>
Co-authored-by: KeithTheEE <kmurrayis@gmail.com>