Canvas source initialization from HTML element

[lbud]

• Modifies CanvasSource#canvas property to accept either a string denoting the ID of the canvas element from which to read (as it previously functioned) or an HTMLCanvasElement (fixes Canvas source initialization from HTML element, instead of element id #6372)
• Introduces the concept of union types to the style spec, denoted similarly to enum types (see canvas)

cc @ryanbaumann

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs (will be autogenerated in mb-pages update)
• manually test the debug page

[jfirebaugh]

Since the canvas source type is inherently a gl-js-specific concept, should we remove it from v8.json entirely? This was first suggested in #5545.

[lbud]

Superseded by #6424.