WIP: Initial draft of new language option for EP settings

[felipeelia]

This draft became too big, so I'm breaking it into smaller pieces:

• WP-CLI get-index-settings
• cached index settings
• display languages in the status report
• stop words
• add default language option
• delete all transients at once during uninstall

Description of the Change

Closes #3302

How to test the Change

Changelog Entry

Added - New feature
Changed - Existing functionality
Deprecated - Soon-to-be removed feature
Removed - Feature
Fixed - Bug fix
Security - Vulnerability

Credits

Props @username, @username2, ...

Checklist:

• 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.

[felipeelia]

To make the process easier, I'm going to break this PR into smaller parts:

• WP-CLI get-index-settings
• cached index settings
• display languages in the status report
• stop words
• add default language option
• delete all transients at once during uninstall