Emoji.Parse should handle emojis without a variation selection

[d4n3436]

Parsing an emoji without a variation selection ('\ufe0f') causes Emoji.Parse and Emoji.TryParse to fail because it expects the emoji string to contain a variation selection. This character is used to specify the type of rendering and Discord requires it in some emojis.

Emoji.Parse/TryParse should handle emoji strings without this character, and should add this character if it's required.