fix printing for single element tuple values (#2786)

adds trailing comma to single element tuple

example:
```carbon
package ExplorerTest api;

fn Main() -> i32
{
  var a: (i32,) = (1, 2);
  return 0;
}

```

change in error message:
```diff
-type error in initializer of variable: '(i32, i32)' is not implicitly convertible to '(i32)'
+type error in initializer of variable: '(i32, i32)' is not implicitly convertible to '(i32,)'
```

explorer/ast/value.cpp

@@ -414,10 +414,14 @@ void Value::Print(llvm::raw_ostream& out) const {
     case Value::Kind::TupleValue: {
       out << "(";
       llvm::ListSeparator sep;
-      for (Nonnull<const Value*> element :
-           cast<TupleValueBase>(*this).elements()) {
+      const auto elements = cast<TupleValueBase>(*this).elements();
+      for (Nonnull<const Value*> element : elements) {
         out << sep << *element;
       }
+      // Print trailing comma for single element tuples: (i32,).
+      if (elements.size() == 1) {
+        out << ",";
+      }
       out << ")";
       break;
     }