feat(query): add 'getObserversCount' util

[DamianOsipiuk]

Adding 'getObserversCount' util

Reson for making this change:

The observers on the query is private property, therefore Typescript is complaining when trying to use it.

DevTools is using the length of the observers in a couple of places like here

Since this property should probably stay as private, I have added utility to get the observers count as a getter method.

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[DamianOsipiuk]

@TkDodo Hi, any chance this could be merged? 😉

[TkDodo]

DevTools is using the length of the observers in a couple of places like here

sure, can you maybe use new function then in the devtools and replace the private accesses with it?

[DamianOsipiuk]

@TkDodo Sure 👍

[TkDodo]

it's also used in tests with ts-expects-error as far as I can see :)

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🎉 This PR is included in version 3.14.0 🎉

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