Add polyfill for PHP8's Stringable

Author: nicolas-grekas

README.md

@@ -1,8 +1,9 @@
 Symfony Polyfill / Php80
 ========================
 
-This component provides functions added to PHP 8.0 core:
+This component provides features added to PHP 8.0 core:
 
+- `Stringable` interface
 - [`fdiv`](https://php.net/fdiv)
 - `ValueError` class
 - `FILTER_VALIDATE_BOOL` constant

Resources/stubs/Stringable.php

@@ -0,0 +1,9 @@
+<?php
+
+interface Stringable
+{
+    /**
+     * @return string
+     */
+    public function __toString();
+}