It defaults to `true` for parameters of type IMAGE, VIDEO, and AUDIO,
and is ignored for any other parameter type.
Example: Using `parameterType.IMAGE` for a parameter `param` in a
plugin named `my-plugin` is equivalent to calling
`jspsych.pluginAPI.registerPreload("my-plugin", "param", "image")`
before `jsPsych.run()`. Setting `preload: false` for the parameter would
disable this invocation.
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.
