assert: fix AssertionError, assign error code

Using `assert.AssertionError()` without the `new` keyword results
in a non-intuitive error:

```js
> assert.AssertionError({})
TypeError: Cannot assign to read only property 'name' of function 'function ok(value, message) {
  if (!value) fail(value, true, message, '==', assert.ok);
}'
    at Function.AssertionError (assert.js:45:13)
    at repl:1:8
    at realRunInThisContextScript (vm.js:22:35)
    at sigintHandlersWrap (vm.js:98:12)
    at ContextifyScript.Script.runInThisContext (vm.js:24:12)
    at REPLServer.defaultEval (repl.js:346:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.onLine (repl.js:545:10)
    at emitOne (events.js:101:20)
>
```

The `assert.AssertionError()` can only be used correctly with `new`,
so this converts it into a proper ES6 class that will give an
appropriate error message.

This also associates the appropriate internal/errors code with all
`assert.AssertionError` instances and updates the appropriate test
cases.

PR-URL: https://github.com/nodejs/node/pull/12651


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>