feat: move Expectations API out of external plugin

tests/Features/Expect/toEqual.php

@@ -0,0 +1,15 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect('00123')->toEqual(123);
+});
+
+test('failures', function () {
+    expect(['a', 'b', 'c'])->toEqual(['a', 'b']);
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    expect('042')->not->toEqual(42);
+})->throws(ExpectationFailedException::class);