Add support for registering customize post meta via register_meta()

[westonruter]

• Allow meta to be registered for the customizer via register_meta()
• Throw a doing_it_wrong if auth_callback is passed among the customize_setting_args.
• Throw a doing_it_wrong if register_meta() was not called with a sanitize_callback.

Example of registering new postmeta along with customizer support. First, given that the post type support has been assigned to the appropriate post types, for example:

add_action( 'init', function() {
    add_post_type_support( 'post', 'greeting' );
}, 1 );

Current (old) way to register postmeta in the customizer:

register_meta( 'post', 'greeting', 'wp_kses_post' );

add_action( 'customize_posts_register_meta', function( WP_Customize_Posts $customize_posts ) {
    foreach ( get_post_types_by_support( 'greeting' ) as $post_type ) {
        $customize_posts->register_post_type_meta( $post_type, 'greeting', array(
            'post_type_supports' => 'greeting',
            'transport' => 'postMessage',
        ) );
    }
} );

New experimental unified way to register postmeta:

register_meta( 'post', 'greeting', array(
    'sanitize_callback' => 'wp_kses_post',

    // Additional props not possible before.
    'type' => 'string',
    'description' => __( 'Greeting', 'my-plugin' ),
    'single' => true,
    'show_in_rest' => true,

    // Extensions to register_meta().
    'show_in_customizer' => true,
    'post_type_supports' => 'greeting',
    'customize_setting_args' => array(
        'transport' => 'postMessage',
    ),
) );

Fixes #192

[westonruter]

Moving this to future release because I don't want to prematurely modify how register_meta is interacted with.