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.
We can't reuse caches from different platform when we compile against
different jar on each platform. Since they have same api we may use
platform specific jars only for runtime.
Zalim Bashorov