Bundle the i18n command from WP-CLI

[bradyvercher]

With the release of WordPress 5.0 and WP.org switching over to WP-CLI's i18n command, support needs to be added for new features and workflows. Bundling the i18n command from WP-CLI without adding any new dependencies should be possible, however there may be breaking changes due to differences in how the WP-CLI command and old PHP tools worked.

This issue will be used to track the changes and hopefully cover features that may break.