remove momentum from list of rollable stats

[rsek]

this would probably require a rewrite of significant portions of the PlayerCharacter class, its related enums, and any commands that touch PCs.

it might be worth following the game's parlance here:

• edge, heart, iron, shadow, and wits are "stats"
• momentum, health, spirit, and supply are all "meters"
• health, spirit, and supply are "condition meters", but momentum is not
• PCs may or may not have additional condition meters from their assets
• only "stats" and "condition meters" are rollable

[rsek]

there's an existing ConditionMeter used by assets in my new dataforged classes, btw, which might be a good starting point if a more structured approach to condition meters is called for.