Add support for WeakRefs

[Matchlighter]

Implement options for making computeds and reactions/autoruns garbage-collectable without manually disposing them.

I've had a couple apps that I've needed to use keepAlive computeds. I'm lazy and can't make promises that future me/future my app will dispose them properly. Now that JS supports WeakRefs, this is a less of a problem. The use case is pretty slim, mainly around computed(() => {}, { keepAlive: true }) stuff, but it can be used for autoruns too (though you'll need to keep a reference to the autorun in some other way less it fall out of scope and get garbage collected).

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

[changeset-bot]

🦋 Changeset detected

Latest commit: c20a32c

The changes in this PR will be included in the next version bump.

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mobx	Minor

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[rvion]

nice, sounds handy !