Fix IReactionDisposer and IIsObservableObject Typing

[barroij]

This is typing only PR.

Current interface definition for IReactionDisposer is

export interface IReactionDisposer {
    (): void
    $mobx: Reaction
}

but that's wrong because because like so typescript considers the property key $mobx to be a string, whereas it is a symbol. The correct indexing type is given by

export interface IReactionDisposer {
    (): void
    [$mobx]: Reaction
}

[Edit] : same thing for IIsObservableObject

[changeset-bot]

🦋 Changeset detected

Latest commit: da624fb

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

This PR includes changesets to release 1 package

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mobx	Patch

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[kubk]

@barroij Thank you. Do you think it makes sense to update IIsObservableObject as well?

mobx/packages/mobx/src/types/observableobject.ts

Line 650 in 13a222e

$mobx: ObservableObjectAdministration

[barroij]

@kubk You are right, I'm gonna change it too.

[barroij]

Note : fixing typing for IIsObservableObject resulted in a test failure (packages/mobx/src/api/tojs.ts:57:28 - error TS2339: Property 'forEach' does not exist on type 'ArrayLike<string | symbol>'.).

I had to change the return type of function ObservableObjectAdministration.ownKeys_ from ArrayLike<string | symbol> to Array<string | symbol> to fix it