Create tree-shakable RealtimeChannels and RealtimePresence modules

[owenpearson]

• depends on Create tree-shakeable BaseClient and Rest modules #1374
• create RealtimeChannels and RealtimePresence classes
• RealtimeChannels class encapsulates creation of RealtimeChannel instances with ability to publish and subscribe
• RealtimePresence, if provided, gives access to the RealtimeChannel.presence interface

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3732

[lawrence-forooghian]

@owenpearson since in #1417 we now have separate BaseRest and BaseRealtime classes, I guess we don't need to create a RealtimeChannels module any more, do we? i.e. we just need to split out realtime presence.

[lawrence-forooghian]

Done in #1428.