Add support for OFFSET with the ROWS keyword

MySQL doesn't support the ROWS part of OFFSET. Teach the parser to
remember which variant it saw, including just ROW.

src/parser.rs

@@ -1970,10 +1970,16 @@ impl Parser {
     }
 
     /// Parse an OFFSET clause
-    pub fn parse_offset(&mut self) -> Result<Expr, ParserError> {
+    pub fn parse_offset(&mut self) -> Result<Offset, ParserError> {
         let value = Expr::Value(self.parse_number_value()?);
-        self.expect_one_of_keywords(&["ROW", "ROWS"])?;
-        Ok(value)
+        let rows = if self.parse_keyword("ROW") {
+            OffsetRows::Row
+        } else if self.parse_keyword("ROWS") {
+            OffsetRows::Rows
+        } else {
+            OffsetRows::None
+        };
+        Ok(Offset { value, rows })
     }
 
     /// Parse a FETCH clause