Introduce a new Script builder class

JavaScript is a common feature of the documentation generated by the standard doclet. However, the code to generate scripts is replicated and distributed across a number of different classes.

Not only is this difficult to maintain, it has also led to needless inconsistencies between the instances, and occasional bugs.

It would be better to encapsulate the logic to generate these scripts, ensuring that all appropriate rules for including JavaScript in HTML documents are followed.