Add isListening method and listeningState listener

[khromov]

As per #43 and #68 there is currently some usability issues caused by differences between iOS and Android API:s where Android will stop listening automatically after a few seconds while iOS will continue listening indefinitely.

This PR aims to solve the problem by allowing users better control over when the speech recognition is ongoing by adding the isListening() method for polling whether speech recognition is currently running, as well as addListener('listeningState', ...) for notifying users when speech recognition starts and stops.

The new API:s as per the readme are:

addListener('listeningState', ...)

addListener(eventName: "listeningState", listenerFunc: (data: { status: 'started' | 'stopped'; }) => void) => Promise<PluginListenerHandle> & PluginListenerHandle

Called when listening state changed.

Param	Type
eventName	'listeningState'
listenerFunc	(data: { status: 'started'

Returns: Promise<PluginListenerHandle> & PluginListenerHandle

Since: 6.0.0

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isListening()

isListening() => Promise<{ listening: boolean }>

Check if the player is currently listening.

Since: 6.0.0