Adds a new `WP_PHPMailer` class to leverage the WordPress i18n system with PHPMailer, so that any user-visible error messages can be properly translated.
Props sukhendu2002, swissspidy, audrasjb, iandunn, nacin, mark-k.
Fixes#23311.
Built from https://develop.svn.wordpress.org/trunk@59592
git-svn-id: http://core.svn.wordpress.org/trunk@58978 1a063a9b-81f0-0310-95a4-ce76da25c4cd