MySQL NOT operator support

- Suppot for NOT operator in MySQL, similarly as in PostgreSQL.
- Add test to verify NOT EXISTS query in MySQL

mysql/operators.go

@@ -0,0 +1,9 @@
+package mysql
+
+import "github.com/go-jet/jet/v2/internal/jet"
+
+// NOT returns negation of bool expression result
+var NOT = jet.NOT
+
+// BIT_NOT inverts every bit in integer expression result
+var BIT_NOT = jet.BIT_NOT