Support converting operands to strings for the + operator

src/com/craftinginterpreters/lox/Interpreter.java

@@ -110,6 +110,14 @@ class Interpreter implements Expr.Visitor<Object> {
                     return (String) left + (String) right;
                 }
 
+                if (left instanceof String) {
+                    return (String) left + stringify(right);
+                }
+
+                if (right instanceof String) {
+                    return stringify(left) + (String) right;
+                }
+
                 throw new RuntimeError(expr.operator, "Operands must be two numbers or two strings.");
             case SLASH:
                 checkNumberOperands(expr.operator, left, right);