Package mention finds the mentions in a message body: `@name` and `@role:R` written
in text, outside code and links. It only reads text; which members a mention names is
decided by the board, against its members when the message is posted.
Find returns the mentions in body, in the order written, repeats included. Code
(fenced blocks and inline code spans) and links are skipped; an "@" is a mention only
where a word starts, so an email address or a "\@" isn't one.