Remove usage of assert.invariant

This reduces dependencies when building for the browser target, since
the assert module is a Node.js dependency.

src/helpers.ts

@@ -30,3 +30,14 @@ export function getClass(prototype: Record<string, any>) {
   PhpClass.prototype = prototype
   return PhpClass
 }
+
+/**
+ * Ensures that the given {@link value} is truthy, throws an {@link Error} otherwise.
+ * @param value the value to check to be truthy.
+ * @param message the message of the {@link Error} if the value is falsy.
+ */
+export function invariant(value: any, message?: string) {
+  if (!value) {
+    throw new Error(message)
+  }
+}

src/serialize.ts

@@ -1,5 +1,4 @@
-import invariant from 'assert'
-import { isInteger, getByteLength } from './helpers'
+import { isInteger, getByteLength, invariant } from './helpers'
 
 function getClassNamespace(item: any, scope: Record<string, any>) {
   return (

src/unserialize.ts

@@ -1,7 +1,6 @@
-import invariant from 'assert'
 // eslint-disable-next-line import/no-cycle
 import Parser from './parser'
-import { isInteger, getClass, getIncompleteClass, __PHP_Incomplete_Class } from './helpers'
+import { isInteger, getClass, getIncompleteClass, __PHP_Incomplete_Class, invariant } from './helpers'
 
 export type Options = {
   strict: boolean