Make Listing component a Global Mixin

[duncanmcclean]

This pull request makes the components/Listing.vue component a global mixin. This means that addon developers no longer need to do imports from vendor directories to use this component.

Instead, now it can be imported as a mixin:

Before:

import Listing from "../../../vendor/statamic/cms/resources/js/components/Listing";

export default {
    components: {
        Listing,
    },
}

After:

export default {
    mixins: [Listing],
}

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This PR closes #2559.