fix(spells): backfill casting times for all imported spells

data/spells/silence.json

@@ -5,6 +5,7 @@
   "range": "long",
   "save": "will",
   "base_dc": 10,
+  "cast_time": "one_round",
   "effects": [],
   "description": "Casting time 1 round. A 20-ft.-radius emanation centered on a creature, object, or point in space: all sound is stopped in the area, spells with verbal components cannot be cast, and creatures inside are immune to sonic and language-dependent effects. Duration 1 round/level. Will negates (object). No combat effect encoded: the engine has no verbal component or sound propagation model."
 }