tools: add find-inactive-collaborators.js

The plan is to eventually call this script with a scheduled GitHub
Action that could automatically open pull requests to move collaborators
to emeritus status after (for example) a year of inactivity.

Sample run:

```
$ node tools/find-inactive-collaborators.mjs '30 months ago'
864 authors have made commits since 30 months ago.
101 landers have landed commits since 30 months ago.
146 reviewers have approved landed commits since 30 months ago.
109 collaborators currently in the project.

Inactive collaborators:

Thomas Watson
$
```

PR-URL: https://github.com/nodejs/node/pull/39262


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>