Add support for installing Pest into a Lumen application

This entails creating Laravel and Lumen-specific stubs, and ensuring
that the appropriate stubs are copied as part of the pest install.

stubs/Lumen/Helpers.php

@@ -0,0 +1,11 @@
+<?php
+
+use Illuminate\Contracts\Auth\Authenticatable;
+
+/**
+ * Set the currently logged in user for the application.
+ */
+function actingAs(Authenticatable $user, string $driver = null): TestCase
+{
+    return test()->actingAs($user, $driver);
+}