refactor(core): merge CoreIsolate and EsIsolate

[bartlomieju]

This PR merges EsIsolate into CoreIsolate.

By default CoreIsolate uses NoopModuleLoader that is a placeholder implementation
of ModuleLoader that errors anytime module load is attempted.

A new intializer has been added CoreIsolate::new_with_loader - replaces EsIsolate::new -
it's required to pass ModuleLoader instance.

TODO:

• bike-shedding - do we want to rename CoreIsolate? Do we want to rename CoreIsolate::new_with_loader()?
• should we add CoreIsolate::new_with_loader_and_heap_limits? Or maybe add heap_limits as 3rd/4th param to CoreIsolate::new() and CoreIsolate::new_with_loader()?

[ry]

Let’s leave it CoreIsolate for now - to avoid too many changes at once - but I’m very open to changing it.

[ry]

LGTM