feat: move Expectations API out of external plugin

tests/Features/Expect/toBeString.php

@@ -0,0 +1,16 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect('1.1')->toBeString();
+    expect(1.1)->not->toBeString();
+});
+
+test('failures', function () {
+    expect(null)->toBeString();
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    expect('42')->not->toBeString();
+})->throws(ExpectationFailedException::class);