Move and split up weather.py.

This is getting out of hand, and I'm about to make it worse.

game/weather/fog.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from game.utils import Distance
+
+
+@dataclass(frozen=True)
+class Fog:
+    visibility: Distance
+    thickness: int