Allow LIKE queries against the 'key' value in meta queries.

The new `compare_key=LIKE` parameter works in conjunction with `key` in a
similar way to the `compare=LIKE` and `value`: by doing a "compares" `LIKE`
query. This allows developers to do partial matches against keys when
doing meta queries.

Props mariovalney, chasewg.
Fixes #42409.

git-svn-id: https://develop.svn.wordpress.org/trunk@42768 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-query.php

@@ -604,6 +604,7 @@ class WP_Query {
 	 *              Introduced `RAND(x)` syntax for `$orderby`, which allows an integer seed value to random sorts.
 	 * @since 4.6.0 Added 'post_name__in' support for `$orderby`. Introduced the `$lazy_load_term_meta` argument.
 	 * @since 4.9.0 Introduced the `$comment_count` parameter.
+	 * @since 5.0.0 Introduced the `$meta_compare_key` parameter.
 	 *
 	 * @param string|array $query {
 	 *     Optional. Array or string of Query parameters.
@@ -640,6 +641,7 @@ class WP_Query {
 	 *     @type int          $m                       Combination YearMonth. Accepts any four-digit year and month
 	 *                                                 numbers 1-12. Default empty.
 	 *     @type string       $meta_compare            Comparison operator to test the 'meta_value'.
+	 *     @type string       $meta_compare_key        Comparison operator to test the 'meta_key'.
 	 *     @type string       $meta_key                Custom field key.
 	 *     @type array        $meta_query              An associative array of WP_Meta_Query arguments. See WP_Meta_Query.
 	 *     @type string       $meta_value              Custom field value.