[BREAK] API updates

[d-gubert]

What? ⛵

Remove deprecated methods and behaviors for version 2.0

Methods that will be removed:

• App.getAppUsername() - Developers should use IRead.getUserReader().getAppUser() instead
• ILivechatRead.getLivechatVisitors(query) - There are alternative methods available in the ILivechatRead interface that allow specific querying of visitors
• IRoomBuilder.addUsername(username) - Replace with addMemberToBeAddedByUsername method of the same interface
• IRoomBuilder.setUsernames(usernames) - Replace with setMembersByUsernames method of the same interface
• IRoomBuilder.getUsernames() - Replace with getMembersUsernames of the same interface
• ILivechatRead.isOnlineAsync() - Replace with isOnline, which will change to be an async method

Events that will be removed:

• ILivechatRoomClosedHandler - Developers should use IPostLivechatRoomClosed instead

Methods that will suffer breaking changes:

• ILivechatRead.isOnline(departmentId?) - Return type will change from boolean to Promise<boolean>
• IUserRead.getAppUser(appId?) - The parameter appId will be removed

Data types that will suffer breaking changes:

• IRoom - property usernames will be removed; To get the members of a room (users inside a room), use the IRoomRead.getMembers() method.

Links 🌎

Need to update dependency on this PR later RocketChat/Rocket.Chat#27696

PS 👀