Handler Class Extension and Method Signature Updates

[vsevex]

Here is a brief overview of the changes that have been made and the beautified code:

1. Handler class Extension: The Handler class now extends EventObject (named as Event<T> in the project). This modification was done to improve the handling of incoming stanzas and to separate result and error stanzas using the Dartz package. With this enhancement, we can now easily categorize stanzas as error and result stanzas.
2. addHandler Method Signature Updates: The addHandler method has been updated to accept two additional parameters: resultCallback and errorCallback. These methods play a crucial role in running various executions based on the type of stanzas received, enabling better control over the handling of results and errors.

As a consequence of these changes, some breaking changes have occurred in the codebase, including the logical changes in extensions like disco, pubsub, and v-card.

Versions Affected:
v0.0.1, v0.0.5, v0.0.55