feat: implement subscription IDs

[OrKoN]

This PR implements refactoring to support subscription IDs. In-scope:

• Internal refactoring to align with the spec.
• Generation of subscription IDs.

To be addressed in follow-ups:

• unsubscribe by IDs
• unit tests should not assert only channels but the entire subscription struct.

Notable changes:

• removal of subscribe priority (the insertion order reflects the priority).
• fix in findTopLevelBrowsingContext to not return an id for non-existing contexts.
• clearer way to handle global events (that are not related to any given context).
• checks like isSubscribed should use specific events instead of module event names.