Update "wp scaffold child-theme" to support the latest theme functions.

[sachyya]

Feature Request

Describe your use case and the problem you are facing
wp scaffold child-theme creates the child theme with the old theme functions like get_template_directory_uri. It would be better if this command supports new functions like get_theme_file_uri which is introduced in 4.7.

Also the theme slug created in functions.php created by the command should have slug of the child theme rather than the parent theme.

Describe the solution you'd like

The command should be updated such that it supports the latest theme functions with backward compatibility.

[sachyya]

#227