Function emoji_query_at_caret 

pub fn emoji_query_at_caret(text: &str, caret: usize) -> Option<EmojiQuery>

The :shortcode ending at caret, if the caret sits in one: a : that opens a token, followed by at most MAX_SHORTCODE_LEN shortcode characters. None inside a word (10:30, a:smile) and for a token that already has its closing : after the typed part.