feat: move Expectations API out of external plugin

tests/Features/Expect/toBeIterable.php

@@ -0,0 +1,23 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect([])->toBeIterable();
+    expect(null)->not->toBeIterable();
+});
+
+test('failures', function () {
+    expect(42)->toBeIterable();
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    function gen(): iterable
+    {
+        yield 1;
+        yield 2;
+        yield 3;
+    }
+
+    expect(gen())->not->toBeIterable();
+})->throws(ExpectationFailedException::class);