Adds automatic grouping to the initiative tracker. This will group
any combatants with the same initiative and the same base actor
into a group.

Groups are displayed as collapsible sections in the combat tracker
that use the name from the base actor's prototype token. It lists
the number of combatants in the group, and when one of the group
members is active it displays how far through the group that actor
is.

Currently only offers the one grouping mode, but could be expanded
in the future to support different grouping such as splitting it
into "friendly" and "hostile" groups.

Closes #3667

Tweak the generated HTML to work with the AppV2 combat tracker in
V13. Also moved the `collapsible` styles so they are usable
without the `dnd5e2` class so long as the `dnd5e2-collapsible`
class is also used.