Upgrade baseModifier fields to support resolvable values

[kanongil]

This is a compliment to #20515 to allow SpecialStatistic baseModifier.{check, mod, dc} fields to support resolvable values. This makes it much more versatile, and enables to create level-dc and merged stats.

To create a level DC:

{
  "key": "SpecialStatistic",
  "slug": "level",
  "label": "PF2E.LevelLabel",
  "baseModifier": {
    "dc": "match(when(lte(@actor.level, 20), 14 + floor(max(@actor.level, 0) * 1.345)), 40 + (min(@actor.level, 25) - 20) * 2)"
  }
}

To support Predictable!:

{
  "key": "SpecialStatistic",
  "slug": "deception-or-hard-level",
  "label": "Deception or Hard Level",
  "baseModifier": {
    "dc": "max(@actor.skills.deception.dc.value, 2 + match(when(lte(@actor.level, 20), 14 + floor(max(@actor.level, 0) * 1.345)), 40 + (min(@actor.level, 25) - 20) * 2))"
  }
}