fix(parsing): SubMysteryGift missing tag during SUBtember

[Larandar]

• Tag msg-param-sender-count is missing during (some?) promotions
• Only seen with sender twitch, therefore handled as an edge case
• If sent by twitch and the tag is missing, then assume u64::MAX

Pull request checklist:

• CHANGELOG.md was updated, if applicable

[Larandar]

See issue #215 for my initial report

[RAnders00]

I propose, since a breaking release is upcoming anyways, to change the datatype toOption<u64>? I find it to be clearer and less error-prone (on the library user side). The documentation on the field should explain when the field is typically set and when it typically isn‘t set (subtember and from Twitch).

[Larandar]

Since I was an outsider contributing, I was reticent to introduce a breaking change. But it is indeed the best way to handle it :)

[RAnders00]

Thank you very much!