Position defines a specific location, identified by paths line numbers and
image coordinates, within a specific diff, identified by start, head and
base commit ids.
Text position will have: new_line and old_line
Image position will have: width, height, x, y
I've recently been using your type definitions while playing around with
the Github Webhooks and I realised the `install_repositories` event was
missing. I've added it and updated the tests.