Imperatively allocate window agents and agent clusters

Previously, the spec vaguely noted that all agents and agent clusters
must be allocated at the appropriate time.

With this change, similar-origin window agents are allocated
imperatively during document and browsing context creation, along with
their corresponding agent clusters. The agent clusters are stored in the
appropriate browsing context group, via a map keyed by scheme and
registrable domain.

Fixes #4361. Further work on imperative agent/agent cluster allocation,
specifically for workers and worklets, is tracked in #4339.