[FEATURE] Add removeExisting Option for Listener Management

[AndrewBarber]

Is your feature request related to a problem? Please describe.
Currently, there's no way to control whether existing listeners should be preserved or removed when using mutationReady on an element multiple times. This can lead to duplicate callbacks.

Describe the solution you'd like
Add an options parameter with removeExisting flag:

mutationReady('.my-element', callback, { removeExisting: true });

This would:

• Remove existing listeners when removeExisting is true
• Keep current behaviour (multiple listeners) when false or undefined

Example Usage

// Clear existing and add new
mutationReady('.element', callback, { removeExisting: true });

// Current behaviour
mutationReady('.element', callback);

Additional context
Useful for SPAs where components might be mounted multiple times.