Add a polyfill for the Attribute class

Code which works with both php 7 and 8 may need to use the class constants or
instances of the class outside of attribute declarations. E.g.

    // This line throws without a polyfill for Attribute.
    const FUNCTIONLIKE_ATTRIBUTES = Attribute::TARGET_FUNCTION | Attribute::TARGET_METHOD;

    #[Attribute(FUNCTIONLIKE_ATTRIBUTES)]
    function example() { }

For #235

Author: TysonAndre

Resources/stubs/Attribute.php

@@ -0,0 +1,22 @@
+<?php
+
+#[Attribute(Attribute::TARGET_CLASS)]
+final class Attribute
+{
+    const TARGET_CLASS = 1;
+    const TARGET_FUNCTION = 2;
+    const TARGET_METHOD = 4;
+    const TARGET_PROPERTY = 8;
+    const TARGET_CLASS_CONSTANT = 16;
+    const TARGET_PARAMETER = 32;
+    const TARGET_ALL = 63;
+    const IS_REPEATABLE = 64;
+
+    /** @var int */
+    public $flags;
+
+    public function __construct(int $flags = Attribute::TARGET_ALL)
+    {
+        $this->flags = $flags;
+    }
+}