Deprecate {golem} JS built-in functions

[ColinFay]

Following discussion with @daattali at #449 and now that {shinyjs} is under an MIT licence, there is no need to maintain built-in JavaScript functions in {golem}, so we'll remove them, and advice to switch for {shinyjs}.

We'll keep the add_js* file and invoke_js() mechanism, as I still think this is in {golem} scope to be able to provide tools for building JavaScript extension when you're comfortable doing it.

[dmenne]

It would be nice to have some documentation on the subject. Migrating to golem worked for my app that uses shinyjs, but can't get Javascript initialized.

This is the same problem as https://github.com/asbates/shinyjsgolem which is still open.