Add $ to surroundingPairs configuration

[hansonchar]

Motivation

VS Code currently has a helpful behavior where, if a block of text is selected and the user presses the backtick ("`"), the selected text is wrapped in backticks rather than replaced.

This PR extends that behavior to the dollar sign ($) for markdown contexts. Specifically, when text is selected and the user presses $, the editor would wrap the selection in $...$, rather than replacing it with a single dollar sign.

Testing

Locally built, installed, and tested the changes. It appears to work as expected.

[adomani]

Relavant Zulip discussion.

[adomani]

In particular, I would rather this be opt-in: I would like this to be in place for latex files, but I would prefer if this were not the default behaviour for lean files.