fix: code deletion when switching between multiple instances

[MrSurana]

The function _handleInitialRevisions() inside firebase-adapter.ts was not ceasing operation when the dispose() method was called. Because the this._zombie flag was set to true only once Ready event was triggered. But since this is the first method that is supposed to initialise the text and then trigger the Ready event, the operation in this method would continue executing even if firepad is disposed.

To prevent this, the this._zombie variable can be used to declare that firepad is now disposed even before triggering the Ready event.

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This discrepancy has always been there in the code since the migration to typescript. Here is the actual firepad implementation of dispose, note that the _handleInitialRevisions method reset is missing from our fork.

https://github.com/FirebaseExtended/firepad/blob/master/lib/firebase-adapter.js#L70