Avoid committing common local system files like .DS_Store and the .idea folder

The addition of a module-specific .gitignore file avoids to mistakenly commit the common macOS file .DS_Store (put all over) and the .idea folder used by PhpStorm to contain IntelliJ’s project specific settings files.

There are more possible contenders to consider for inclusion, but these two are the most commonly erroneous committed files that cover the large majority. It does avoid cleanup tasks when more developers get added as module maintainers.