Fix ReservedSql type to extend ISql instead of Sql

ReservedSql incorrectly extends Sql, exposing methods like begin(),
listen(), subscribe(), reserve(), end(), etc. that do not exist at
runtime on reserved connections. Change it to extend ISql, matching
the approach used for TransactionSql.

Fixes #713

Author: macrosak

types/index.d.ts

@@ -727,7 +727,7 @@ declare namespace postgres {
     prepare<T>(name: string): Promise<UnwrapPromiseArray<T>>;
   }
 
-  interface ReservedSql<TTypes extends Record<string, unknown> = {}> extends Sql<TTypes> {
+  interface ReservedSql<TTypes extends Record<string, unknown> = {}> extends ISql<TTypes> {
     release(): void;
   }
 }