Add WP CLI bash script to migrate tutorials to lessons

[adamwoodnz]

Includes 2 scripts:

1. bin/wp-cli-tutorial-to-lesson-conversion-script.sh changes the post type of the existing tutorial to a lesson
2. bin/wp-cli-tutorial-to-new-lesson-script.sh creates a new lesson, copying across post data, and saves the tutorial as a draft

Both include dry run mode and file mode for batch operations.

I haven't tested either of these yet, I've simply created them to see if each approach is possible and to compare.

See #2393