Feature request: implement onShow and onHide callbacks on web

[Frank3K]

Description

The Intercom JavaScript API provides two callback methods that allow developers to trigger events when the messenger is shown or hidden:

onShow

This callback fires when the Intercom messenger is shown. It accepts a function as an argument.
Intercom API Docs: onShow

onHide

This callback fires when the Intercom messenger is hidden. It accepts a function as an argument.
Intercom API Docs: onHide

Feature Request

Could you consider adding support for these methods in intercom_flutter? As far as I can tell, these methods are currently only available for the web platform.

Thank you for maintaining this package—it's been working well in my tests (on web).

[deepak786]

Maybe related #443