feat: make receiver in Tact template refund remaining value

Deploy returns excessive funds when the user uses `@stdlib/deploy` and
`Deployable` trait. Therefore, it would be nice to showcase how to do
the same in `receive()` functions right in the counter template.

Related links:
* tact-lang/tact-docs#231
* tact-lang/tact-docs#232

src/templates/tact/counter/contracts/contract.tact.template

@@ -17,6 +17,9 @@ contract {{name}} with Deployable {
 
     receive(msg: Add) {
         self.counter += msg.amount;
+
+        // Notify the caller that the receiver was executed and forward remaining value back
+        self.notify("Cashback".asComment());
     }
 
     get fun counter(): Int {