feat: move Expectations API out of external plugin

tests/Features/Expect/toBeCallable.php

@@ -0,0 +1,18 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect(function () {})->toBeCallable();
+    expect(null)->not->toBeCallable();
+});
+
+test('failures', function () {
+    $hello = 5;
+
+    expect($hello)->toBeCallable();
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    expect(function () { return 42; })->not->toBeCallable();
+})->throws(ExpectationFailedException::class);