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.