fix: add CONSTRAINT, DOCUMENTATION to keyword; remove unreachable VARIABLE

Author: engalar

mdl/grammar/MDLParser.g4

@@ -3620,7 +3620,7 @@ keyword
     | BEGIN | END | IF | ELSE | ELSIF | ELSEIF | THEN | WHILE | LOOP
     | BREAK | CONTINUE | THROW | RAISE | CASE | WHEN
     | CALL | LOG | TRACE | WITH | FOR | TO | OF | RETURNING | RETURNS
-    | NOTHING | EXPRESSION | JAVASCRIPT | VARIABLE
+    | NOTHING | EXPRESSION | JAVASCRIPT
 
     // Query / SQL
     | SELECT | FROM | WHERE | JOIN | LEFT | RIGHT | INNER | OUTER | FULL | CROSS