Ignore .ddev folder on composer builds

Problem/Motivation

Adding a .ddev folder to a contrib module exists purely for maintainers and contributors to have a way to easily spin up a local standalone environment when cloning from the module repository directly. But presence of the .ddev folder in package builds intended for real Drupal websites can be confusing and can lead to accidental ddev environment spin ups of the module instead of the website project, which is not recommended and can have unintended consequences.

Proposed resolution

Exclude .ddev folder from composer packages using the gitattributes export-ignore approach outlined here:

https://stackoverflow.com/a/17069547/413538

Alternatively, have Drupal.org packaging maintainers to exclude .ddev folders for all contrib project builds.