Add global listing type / global search

index.php

- Add a new listing type, and set it's "slug" to "global".
- In the Explore page, using Elementor, set this new listing type as the first option in the "Types" tab
- In my-listing/includes/integrations/wp-job-manager/wp-job-manager-queries.php around line 235,
replace this:
```
$args['meta_query'][] = [
    'key'     => '_case27_listing_type',
    'value'   =>  $listing_type,
    'compare' => '='
];
```
with this:
```
if ( $listType->get_slug() !== 'global' ) {
    $args['meta_query'][] = [
        'key'     => '_case27_listing_type',
        'value'   =>  $listing_type,
        'compare' => '='
    ];
}
```