Upgrade TwitchLib.PubSub and move to a submodule

[teknogeek]

This upgrades TwitchLib.PubSub to the latest version, as well as moving it to a git submodule instead of an external dependency.

This upgrade fixes the following warnings shown during build:

streamdeck-chatpager\Twitch\TwitchPubSubManager.cs(133,13,133,36): warning CS0618: 'TwitchPubSub.OnRewardRedeemed' is obsolete: 'This event fires on an undocumented/retired/obsolete topic. Consider using OnChannelPointsRewardRedeemed'
streamdeck-chatpager\Twitch\TwitchPubSubManager.cs(157,13,157,36): warning CS0618: 'TwitchPubSub.OnRewardRedeemed' is obsolete: 'This event fires on an undocumented/retired/obsolete topic. Consider using OnChannelPointsRewardRedeemed'
streamdeck-chatpager\Twitch\TwitchPubSubManager.cs(210,13,210,51): warning CS0618: 'TwitchPubSub.ListenToBitsEvents(string)' is obsolete: 'This topic is deprecated by Twitch. Please use ListenToBitsEventsV2()'
streamdeck-chatpager\Twitch\TwitchPubSubManager.cs(213,13,213,48): warning CS0618: 'TwitchPubSub.ListenToRewards(string)' is obsolete: 'This method listens to an undocumented/retired/obsolete topic. Consider using ListenToChannelPoints()'