Drupal "scaffold" files now duplicated in drupal/core

In order to be able to add Composer-ready project templates to Drupal core, it is first necessary to provide a list of scaffold files in the drupal/core project. This will allow the Scaffold component to place these files in their original location so that a project based on the subtree split of drupal/core may still assemble a complete and functional Drupal site. This change means that these files will exist in two locations, and any change to any of these files must be made in both places. Unit tests have been added to ensure that the contents of these files remain in alignment.

The complete list of scaffold files is:

.csslintrc
.editorconfig
.eslintignore
.eslintrc.json
.gitattributes
.ht.router.php
.htaccess
INSTALL.txt
README.txt
example.gitignore
index.php
robots.txt
update.php
web.config
sites/README.txt
sites/development.services.yml
sites/example.settings.local.php
sites/example.sites.php
sites/default/default.services.yml
sites/default/default.settings.php
modules/README.txt
profiles/README.txt
themes/README.txt