Allow for modules to extend or duplicate the system's concentration automation

[krbz999]

Related to #3028.

It would be good if whatever method is created to track concentration was able to be extended such that, for example, an actor could

• track concentration on two items at the same time.
• have one 'type' of concentration that is disconnected from regular concentration (example: the Witch class from Valda's Spire of Secrets, which tracks concentration on their hexes separately from spells).

While not part of any official material, it would be good if we made an effort not to directly prevent a module's ability to do this, or better yet exposed the relevant API such that a module could duplicate the automation.

[etiquettestartshere]

While it may not be official official, the Moon Domain Cleric sublcass from Tal’Dorei Campaign Setting Reborn (available on dndbeyond) has "Mind of Two Moons", a divinity feature that allows for a second 'type' of concentration. It would be nice if this feature was supported, or at least not disallowed.

[JPMeehan]

MCDM talent is another example of multi-concentration, as is the official Niv-Mizzet creature.