feat: move Expectations API out of external plugin

tests/Features/Expect/toBeArray.php

@@ -0,0 +1,16 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect([1, 2, 3])->toBeArray();
+    expect('1, 2, 3')->not->toBeArray();
+});
+
+test('failures', function () {
+    expect(null)->toBeArray();
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    expect(['a', 'b', 'c'])->not->toBeArray();
+})->throws(ExpectationFailedException::class);