Add extend variant of match_brackets command.

[loralb]

While trying to create a key mapping with a kakoune-like select mode, I noticed how match_brackets was missing the extend_* variant, so I added it.

[axelcool1234]

I am considering implementing something similar for ma and mi in order to resolve #12927. Would that be of interest? I apologize if this isn't the appropriate place to ask this.