Add conditional tag support for wp_enqueue_script functions

[Otto42]

The style enqueueing functions support the use of conditionals for adding IE-style comment conditionals around stylesheet declarations, like so:

wp_enqueue_style( 'twentytwelve-ie', get_template_directory_uri() . '/css/ie.css', array( 'twentytwelve-style' ), '20121010' );
$wp_styles->add_data( 'twentytwelve-ie', 'conditional', 'lt IE 9' );

This same support should be migrated to the javascript enqueueing system, so as to allow specific javascripts to be loaded only in IE.

This would allow the declaration of html5.js to be properly enqueued instead of hardcoded into the header.php of twentyeleven, twentytwelve, and twentythirteen.

[SergeyBiryukov]

Duplicate of #16024.