Add $ to surrounding pairs in markdown

[LemmusLemmus]

This pull request simply adds the dollar sign ($) to the list of surrounding pairs, which makes it possible to select text, press the dollar sign key and have the text be automatically surrounded with $. This is primarily useful for inline ( $ ) and block ( $$ ) mathematical expressions. The pull request fixes issue #233980 _{(Yes, it is my issue. Last time I tried to submit a new surrounding pair for markdown it did not get added until someone opened an issue).}