[WIP] Scaffold assets

[greg-1-anderson]

This is the code used to produce greg-1-anderson/drupal-scaffold-assets. See greg-1-anderson/drupal-drupal-composer for motivation and comparison with drupal-composer/drupal-project.

The .yml file that goes with the code is:

packages:
  greg-1-anderson/drupal-scaffold-assets:
    source: https://git.drupalcode.org/project/drupal.git
    target: https://github.com/greg-1-anderson/drupal-scaffold-assets.git
    git:
      author:
        name: Greg Anderson
        email: greg.1.anderson@greeknowe.org
    builder: \PackageGeneratorDrupal\DrupalScaffoldAssetsBuilder
    composer:
      metadata:
        name: greg-1-anderson/drupal-scaffold-assets
        type: library
        description: Scaffold assets from drupal/drupal
        license: GPL-2.0-or-later
      file-mapping:
        "[web-root]/.csslintrc": "assets/csslintrc"
        "[web-root]/.editorconfig": "assets/editorconfig"
        "[web-root]/.eslintignore": "assets/eslintignore"
        "[web-root]/.eslintrc.json": "assets/eslintrc.json"
        "[web-root]/.gitattributes": "assets/gitattributes"
        "[web-root]/.ht.router.php": "assets/ht.router.php"
        "[web-root]/.htaccess": "assets/htaccess"
        "[web-root]/example.gitignore": "assets/example.gitignore"
        "[web-root]/index.php": "assets/index.php"
        "[web-root]/robots.txt": "assets/robots.txt"
        "[web-root]/update.php": "assets/update.php"
        "[web-root]/web.config": "assets/web.config"
        "[web-root]/sites/default/default.services.yml": "assets/default.services.yml"
        "[web-root]/sites/default/default.settings.php": "assets/default.settings.php"
        "[web-root]/sites/example.settings.local.php": "assets/example.settings.local.php"
        "[web-root]/sites/development.services.yml": "assets/development.services.yml"
        "[web-root]/sites/default/settings.php":
            shadow: "[web-root]/sites/default/default.settings.php"
            path: "assets/default.settings.php"
            overwrite: false
        "[web-root]/sites/example.sites.php": "assets/example.sites.php"
        "[web-root]/modules/README.txt": "assets/modules.README.txt"
        "[web-root]/profiles/README.txt": "assets/profiles.README.txt"
        "[web-root]/sites/README.txt": "assets/sites.README.txt"
        "[web-root]/themes/README.txt": "assets/themes.README.txt"
        "[web-root]/INSTALL.txt": "assets/INSTALL.txt"
        "[web-root]/README.txt": "assets/README.txt"

Requires webflo/package-generator#1.

You will not necessarily want to take this PR, but it might be useful for supporting the workflow from the Composer in Core initiative in Drupal 8.7.x and earlier, if desired.