Define monster numbers in encounters based on players and sidekicks

[Mike Pirnat]

Encounters in Dragons of Icespire Peak and some of the other new adventures scale based on the number of members in the party--sometimes just players, sometimes including sidekicks--so we should add support for the use of "players" and "sidekicks" as special terms in the encounter monster group count expressions.

And if they don't already, those expressions should support multiplication, addition, and subtraction, so that we can do things like:

[groups.group1]
count = "2 * players"

[groups.group2]
count = "players + sidekicks"

[groups.group3]
count = "players + sidekicks - 1"

[Mike Pirnat]

We can not only base monster counts on the number of players, sidekicks, and overall party members, we can now do full math, like this:

count = "(players - 2) * 3 + (sidekicks / 2) + goblins"

I think our work here is done!