Code Quality: Add @phpstan-assert-if-true to is_wp_error().

This ensures PHPStan is aware that passing a `WP_Error` instance to `is_wp_error()` will cause the function to return `true`. This reduces the total number of PHPStan errors at rule level 7 from 14,271 to 13,233 (-1,038 or -7.27%). 

Developed in #11095

See #64238.


git-svn-id: https://develop.svn.wordpress.org/trunk@61774 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/load.php

@@ -1798,6 +1798,8 @@ function wp_doing_cron() {
  *
  * @param mixed $thing The variable to check.
  * @return bool Whether the variable is an instance of WP_Error.
+ *
+ * @phpstan-assert-if-true WP_Error $thing
  */
 function is_wp_error( $thing ) {
 	$is_wp_error = ( $thing instanceof WP_Error );