Typing support for built-in middleware handlers

[filmaj]

This issue is spun out of #242, which asked for both better built-in middleware listener functions as well as documentation. Splitting the docs vs. typing support into separate issues for easier management. Adding stricter typing to middleware arguments is a breaking change as well.

• What would better typing look like for built-in middleware functions? See comments in directMention() helper function (documentation) #242 for clues.
• Any future change to the types of built-in middleware functions should also include documentation improvements.