Support the string concat operator (#178)

The selected precedence is based on BigQuery documentation, where it is equal to `*` and `/`:

https://cloud.google.com/bigquery/docs/reference/standard-sql/operators

tests/sqlparser_common.rs

@@ -665,6 +665,21 @@ fn parse_in_subquery() {
     );
 }
 
+#[test]
+fn parse_string_agg() {
+    let sql = "SELECT a || b";
+
+    let select = verified_only_select(sql);
+    assert_eq!(
+        SelectItem::UnnamedExpr(Expr::BinaryOp {
+            left: Box::new(Expr::Identifier(Ident::new("a"))),
+            op: BinaryOperator::StringConcat,
+            right: Box::new(Expr::Identifier(Ident::new("b"))),
+        }),
+        select.projection[0]
+    );
+}
+
 #[test]
 fn parse_between() {
     fn chk(negated: bool) {