General: Exclude modifier letters (lm) from user names if strict

Modifier letters are somewhat like combining characters; there's nothing
inherently wrong about using them in usernames, but if we're string and
exclude combining characters, then excluding modifiers makes sense.

Author: arnt

src/wp-includes/formatting.php

@@ -2153,7 +2153,7 @@ function sanitize_user( $username, $strict = false ) {
 
 	// If strict, remove reduce to letters and numbers.
 	if ( $strict ) {
-		$username = preg_replace( '|[^a-z0-9 _.\-@\p{L}\p{N}]|iu', '', $username );
+		$username = preg_replace( '|[^a-z0-9 _.\-@\p{Ll}\p{Lu}\p{Lo}\p{N}]|iu', '', $username );
 	}
 
 	$username = trim( $username );