Add script attribution data

[adamsilverstein]

• Create a new data source that provides a JSON object mapping all enqueued scripts to the plugin that enqueued them.
• Cover scripts added with the Hooks API as well as script tags output directly via the wp_head and wp_footer action callbacks.
• CSS attribution will follow a similar pattern and can be worked on separately.

[adamsilverstein]

See WordPress/performance#1629 for approach to getting script attribution data