The `run()` method takes a timeline and returns a promise that is
resolved when the experiment finishes.
Hence, jsPsych can now be initialized without starting an experiment.
This re-enables usage of `jsPsych.timelineVariable()` in timeline
definitions and repairs exclusion checks and extension loading.
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.
