Editorial: rigorize tasks and task queuing

This introduces tasks as a proper struct, with well-defined fields, and
makes "queue a task" and "queue a microtask" into more well-defined
algorithms for setting up and queuing those tasks. It centralizes and
calls out the still-somewhat-vague parts in the "implied event loop" and
"implied document" definitions.

Other minor cleanups and clarifications:

* Fixes "performing a microtask checkpoint" to be a boolean; previously
  it was referred to as a flag but set to true or false.
* Fixes #4242 by making it clearer how tasks are chosen by the event
  loop.
* Adds more guidance text around how task sources and task queues
  interact.
* Introduces a new subsection "Queuing tasks" to contain the
  task-queuing definitions, which were previously spread out through
  "Definitions" (for non-microtask tasks) and "Processing model" (for
  microtasks).
* Centralizes all properties of the event loop into "Definitions". They
  were previously spread out throughout "Definitions" and "Processing
  model".