Make all agent allocation imperative

Closes #5210.

Closes #4339 by removing the explicit phrasing of a parallel execution
environment, instead relying on the agent infrastructure.

Closes #4988 by reintroducing a definition for "similar-origin window
agent", separate from its creation algorithm. Like the definition of all
agent types, it is now informal, just stating what globals are contained
in the agent.

Relocates the site-related definitions to the "Origin" section of the
spec, adjacent to the "schemelessly same site" and "same site"
definitions.

Exposes some of the higher-level concepts here to the dev edition.