perf: Context independent, shared null && undefined

Currently `null` and `undefined` are created per `JavascriptLibrary`.

- We can't compare a `JsVar` to `null` or `undefined` because we don't know its' origin.
- `JsVar` -> ToRecyc..Object -> Compare... is unnecessarily more complicated
- There are some places we know `JsVar` is actually a `Recyc..Object` hence things are easier (this improvement is already merged to master). Yet, those places are very limited

Any objections to making `null`, `undefined` values are allocated per framework only once?

Is there anything else that we could use the same approach?