Add Scheduler class

[krispya]

This creates a Scheduler class as a web-friendly API, wrapping the functional API. It currently isn't exported.

A couple of notes:

• The schedule returned by the class is fully compatible with the functional API.
• I did not include getTag, hasTag, getRunnable, etc, as methods since you have access to the maps on the schedule directly. We can add them if it makes sense.
• Chaining is supported.
• add uses an options object that gets converted to an array of options functions internally.

To discuss:

• How to export? It can just be its own export so treeshaking is supported.

[krispya]

I removed auto-tag creation from this PR in favor of isolating it to this one: #5