Generate unique enqueue handle based on URL

[swashata]

Right now, it is very much possible that a same file remains is under different entry-points for a particular app. Webpack chunk splitting works this way and creates shared chunks to prevent duplicate.

When this is the case, if we were to enqueue two different entry-points with same chunk, it will be enqueued twice.

One way to prevent it is by defining $handle programmatically (thereby remove user access).

It should be based on following:

1. $appName.
2. $dir ($source).
3. $js URL (relative URL like app/main.js, app/common.js).

We can combine three and create a string which should be same regardless of entry-point. wp_enqueue_script and wp_enqueue_style will handle it.