Extending Unicode support in email addresses, usernames, and slugs.

A request for comment on extending Unicode support in WordPress for email addresses, usernames, and slugs. WordPress currently restricts email addresses to US-ASCII characters, but modern email providers and standards (RFC 6530) now support full UTF-8. The proposal introduces a new WP_Email_Address class and updated functions (wp_is_unicode_email, wp_sanitize_unicode_email) to handle Unicode email addresses. Key concerns include database compatibility (utf8mb4 required), risks from confusable characters and Unicode normalization, and the need to audit existing plugins and core code that assumes ASCII-only email strings. PHPMailer in WordPress 6.9 already supports sending to Unicode addresses; this proposal focuses on storing and validating them.