Instead of a global `jsPsych` object, there is a `JsPsych` class now
which can be instantiated with the parameters that were previously
accepted by `jsPsych.init`. Upon instantiation, the `JsPsych` class also
instantiates classes for stateful modules (`jsPsych.data` and
`jsPsych.pluginAPI`) and passes relevant `JsPsych` instance information
to them. Plugins are expected to be classes now that are instantiated
for each trial and receive the `JsPsych` instance as their only
constructor argument.
* Convert jsPsych and plugins to ES6 modules
* Create packages for each plugin, the core library, and config files
* Setup rollup, jest, and babel configurations
* Update jsPsych to load plugin objects directly from the `type` field
* Update tests to use the ES6 modules
