This PR starts exploring the Page class and how to best document it. It explores how best to document events in the system, and I think pulling them out into an `enum` is the best solution here. It lets us end up with a page of docs that explicitly lists all the events the page class can ever emit.
Just one was used externally and I wrapped that up in a method. I think
it's a useful method to provide (I can imagine wanting to know if JS is
enabled on a page) so I think there's no harm here (I'd rather that then
have JSHandle reach into a private variable).
This adds the new docs that are generated by API Extractor via TSDoc.
They are sparse because we do not yet use TSDoc in the codebase. This is
the starting point and from here we can work through all the different
classes and improve the documentation.
