fix(interpreter): catch KeyError instead of IndexError

src/interpreter/environment.py

@@ -17,7 +17,7 @@ class Environment:
     def get(self, name: Token) -> Any:
         try:
             return self.values[name.lexeme]
-        except IndexError:
+        except KeyError:
             if self.enclosing is not None:
                 return self.enclosing.get(name)
             raise PebbleRuntimeError(name, f"Undefined variable '{name.lexeme}'.")