Add support for the WordPress.org plugin preview

Description of the Change

WordPress.org recently launched support for plugin previews utilizing the WordPress Playground feature. Plugins wanting to take advantage of this need to opt in by setting up a blueprint.json file that configures how the preview should load. This PR adds in that file that does the following:

Sets up an environment running PHP 7.4 (our supported minimum) and the latest version of WordPress
Logs into the admin
Installs and activates the plugin
Imports some test content
Sends the user to a post that has an SVG image

Note that this PR is targeted to trunk so we can take advantage of our Plugin Asset Update Action to deploy these changes without having to push out a new release. This is blocked until https://github.com/10up/action-wordpress-plugin-asset-update/pull/54 is released.

Also note once these changes are on .org, the preview button will need to be enabled in a test state. Once verified as working, we can enable it for all users.

How to test the Change

The WordPress Playground allows you to spin up a new environment directly through the URL, by going to https://playground.wordpress.net/# and pasting your JSON config after the #. In this case, the URL should be: https://playground.wordpress.net/#{%20%22$schema%22:%20%22https://playground.wordpress.net/blueprint-schema.json%22,%20%22landingPage%22:%20%22\/wp-admin\/post.php?post=6&action=edit%22,%20%22preferredVersions%22:%20{%20%22php%22:%20%227.4%22,%20%22wp%22:%20%22latest%22%20},%20%22phpExtensionBundles%22:%20[%22kitchen-sink%22],%20%22steps%22:%20[%20{%20%22step%22:%20%22login%22,%20%22username%22:%20%22admin%22,%20%22password%22:%20%22password%22%20},%20{%20%22step%22:%20%22installPlugin%22,%20%22pluginZipFile%22:%20{%20%22resource%22:%20%22wordpress.org\/plugins%22,%20%22slug%22:%20%22safe-svg%22%20},%20%22options%22:%20{%20%22activate%22:%20true%20}%20},%20{%20%22step%22:%20%22importFile%22,%20%22file%22:%20{%20%22resource%22:%20%22url%22,%20%22url%22:%20%22https:\/\/raw.githubusercontent.com\/10up\/safe-svg\/ec6071406de403e9c9f4b5833abf4e438816df05\/.wordpress-org\/blueprints\/demo-data.xml%22%20}%20}%20]%20}

Changelog Entry

Added - Support for the WordPress.org plugin preview

Credits

Props @dkotter

Checklist:

[x] I agree to follow this project's Code of Conduct.
[ ] I have updated the documentation accordingly.
[ ] I have added tests to cover my change.
[ ] All new and existing tests pass.

10up / safe-svg