support for custom autoexpanding text abbreviations

[matrixbot]

Created by @ matthew:matrix.org.

eg !vw -> !github create vector-im/vector-web

[sbts]

Even Better if you can use it to trigger a user prompt widget for part of the expansion.
And have a couple of built in variable expansions such as
$username
$nick
$bridgeUserName
$bridgeNick
$bridgeNetwork
$roomID
$roomName
eg: expand "idme" to the result of
/msg nickserv ident $bridgeNick %getInput "Enter Password for $bridgeNick on $bridgeNetwork : "

[robintown]

Duplicate of #19747 (that issue has more conversation)