Add --purge flag to control removal of obsolete translations and comments in update-po#460

Copilot wants to merge 6 commits intomainwp-cli/i18n-command:mainfrom

copilot/fix-1570774-111592377-5ffacbc1-5e8a-4e4c-b0c9-3ffb5c94276fwp-cli/i18n-command:copilot/fix-1570774-111592377-5ffacbc1-5e8a-4e4c-b0c9-3ffb5c94276fCopy head branch name to clipboard