feat: move Expectations API out of external plugin

tests/Features/Expect/toBeJson.php

@@ -0,0 +1,17 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect('{"hello":"world"}')->toBeJson();
+    expect('foo')->not->toBeJson();
+    expect('{"hello"')->not->toBeJson();
+});
+
+test('failures', function () {
+    expect(':"world"}')->toBeJson();
+})->throws(ExpectationFailedException::class);
+
+test('not failures', function () {
+    expect('{"hello":"world"}')->not->toBeJson();
+})->throws(ExpectationFailedException::class);