feat: move Expectations API out of external plugin

tests/Features/Expect/toContain.php

@@ -0,0 +1,19 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('passes strings', function () {
+    expect([1, 2, 42])->toContain(42);
+});
+
+test('passes arrays', function () {
+    expect('Nuno')->toContain('Nu');
+});
+
+test('failures', function () {
+    expect([1, 2, 42])->toContain(3);
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    expect([1, 2, 42])->not->toContain(42);
+})->throws(ExpectationFailedException::class);