refactor: node::Environment self-cleanup

[ckerr]

Description of Change

• NodeBindings::CreateEnvironment() now returns a std::shared_ptr<node::Environment> instead of a raw pointer. The new shared pointer has a custom deleter that knows how to cleanup both the node::Environment and its node::IsolateData.
• Remove duplicated cleanup code from ElectronRenderClient, WebWorkerObserver, and NodeEnvironment, which all had code similar to the new deleter.
• Removed now-unused class NodeEnvironment.
• side refactor: since only NodeBindings uses kElectronContextEmbedderDataIndex, make it a private field.

The initial motivation for this PR was to fix a dangling raw_ptr bug in NodeEnvironment; but when investigating the code, it looks like there's an opportunity to reduce coupling across the above classes and remove some code duplication.

note: I'd particularly like revewer 👀 on the changes in NodeBindings.cc, e.g. is the v8::Context being handled correctly when it's passed to the deleter lambda?

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none.

[release-clerk]

No Release Notes

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I was unable to backport this PR to "26-x-y" cleanly;
you will need to perform this backport manually.

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I have automatically backported this PR to "27-x-y", please check out #39628