src: kill off node_isolate

[bnoordhuis]

Continued from #6897. If node wants to grow multi-isolate powers, then node_isolate needs to go. In most places you can trivially replace it with args.GetIsolate() but not everywhere.

Performance impact: args.GetIsolate() is eminently inline-able so the impact should be close or equal to zero.

/cc @indutny

[indutny]

And here #1 comes: #6901

[indutny]

It is rather problematic to do it fully now, but I'll do my best to at least remove it in all obvious places.

[indutny]

And just to record the idea: env->Throw(...) and friends.