The main advantage of this is that we can use a newer and official build of V8.
Also, with new infra, we can use other JS engines.
Other changes:
* ScriptEngine API is simplified and documented.
* Introduce ScriptEngineWithTypedResult with typed `eval`, mostly for JsReplEvaluator and JsScriptEvaluator.
* J2V8 version is completely removed.
* Use new ScriptEngineV8 everywhere by default.
* System property `kotlin.js.useNashorn` switches to Nashorn in all tests.