Introduce file ignorance for Composer distributions of packages #5070
Description
Each of the repos for WP-CLI commands contain their own .distignore file (example).
When a package is sent to Composer's Packagist, this file isn't used, because Packagist uses the Source code ZIP that GitHub builds for each release, and that uses git archive which doesn't read .distignore and uses .gitattributes instead.
In order to ignore unwanted files in Composer distributions, a .gitattributes file should be introduced which contains a list of files to ignore with a export-ignore property (example).
What would be the best way to introduce this file to all command repos?