PR additions/deletions badge

:clipboard: **Description**

GitHub PR additions/deletions badge. Since there are two data to be displayed, I suggest a badge like this:

:link: **Data**

Can be accessed via GitHub [PR API](https://api.github.com/repos/badges/shields/pulls/3295) (last few lines):

```json
"commits": 1,
"additions": 85,
"deletions": 54,
"changed_files": 5
}
```

:microphone: **Motivation**

To show that our PRs are big/small

The issue names the GitHub pull-request API endpoint and its additions and deletions fields; start by inspecting that response and how existing GitHub-backed badges are organized. Done means a badge displays both values in the proposed format, with the behavior verified by the relevant project checks.

Written by the indexing model from the issue text.