Change SchematicsSymbol to SchematicsGear

Other symbols aren't relevant for the challenge, so I'll just rename
this, and add an extra property for it.

3/3_common.ts

@@ -200,11 +200,15 @@ export type EnginePart = {
 };
 
 /**
- * A symbol on the schematics map
+ * A gear on the schematics map
+ * 
+ * A gear is any `*` symbol from the schematics map that borders
+ * exactly two engine part numbers.
  */
-export type SchematicsSymbol = {
+export type SchematicsGear = {
     Symbol: string,
     Coordinates: Vector2,
+    PartNumbers: number[]
 };
 
 /**