feat: move Expectations API out of external plugin

tests/Features/Expect/toBeObject.php

@@ -0,0 +1,16 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect((object) ['a' => 1])->toBeObject();
+    expect(['a' => 1])->not->toBeObject();
+});
+
+test('failures', function () {
+    expect(null)->toBeObject();
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    expect((object) 'ciao')->not->toBeObject();
+})->throws(ExpectationFailedException::class);