Adds stubs; cleanup

lark-stubs/exceptions.pyi

@@ -9,6 +9,10 @@ class LarkError(Exception):
     pass
 
 
+class ConfigurationError(LarkError, ValueError):
+    pass
+
+
 class GrammarError(LarkError):
     pass
 

lark/exceptions.py

@@ -14,9 +14,6 @@ class ConfigurationError(LarkError, ValueError):
 class GrammarError(LarkError):
     pass
 
-class GrammarError_Value(LarkError):
-    pass
-
 
 class ParseError(LarkError):
     pass