Add an interface for drivers that support JavaScript

[jhedstrom]

There is no current way to tell if a driver will support JavaScript without doing a try/catch, which is the proposed approach over in jhedstrom/drupalextension#325

It would be nice if instead an interface was available to check for.

[aik099]

How does Behat knows that when you use @javascript annotation?

[jhedstrom]

I think Behat just uses whatever is configured as the javascript_session in behat.yml...