Add review generator

[nikmclaughlin]

All Submissions:

• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds a generator class and CLI command for generating customer product reviews.

The generator expects products and customers to already exist. It inserts each new review comment on a random existing product and assigns it to a random existing user.

I included an array of ~50 fake review snippets to use as seed data. Using review-like text helped for my purposes, but could be substituted for something from Faker or just generic lorem ipsum if that's preferred. The existing text is also English-only, which may or may not be ideal.

Lastly, it may also be nice to add a flag to trigger review generation during product generation, but that's left for a future enhancement here.

How to test the changes in this Pull Request:

1. Check out the CLI doc, make sure it looks right: wp help wc generate reviews
2. On a site with customers and products, try generating some product reviews, and check the work in the WC Admin UI:
   wp wc generate reviews
wp wc generate reviews 25
3. Try the same on a fresh site with no products or no customers and see that generation fails gracefully

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Add a new generator for product reviews.

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.