Support ALTER TABLE DROP PRIMARY KEY (#682)

* parse alter table drop primary key

* cargo nightly fmt

* add Dialect validation

src/parser.rs

@@ -3137,6 +3137,10 @@ impl<'a> Parser<'a> {
                     name,
                     cascade,
                 }
+            } else if self.parse_keywords(&[Keyword::PRIMARY, Keyword::KEY])
+                && dialect_of!(self is MySqlDialect | GenericDialect)
+            {
+                AlterTableOperation::DropPrimaryKey
             } else {
                 let _ = self.parse_keyword(Keyword::COLUMN);
                 let if_exists = self.parse_keywords(&[Keyword::IF, Keyword::EXISTS]);