Custom notifications based on output

I'm migrating to VS Code's terminal and glad to have found this extension. I was previously using [this method](https://blog.liplex.de/native-osx-notifications-from-iterm-with-triggers/) in iTerm2 which allows having a custom notification show based on certain output in the terminal.

For example, when running unit tests, I have the results in the notification using regex:
Regex: `(TOTAL: (((\\d+)\\b FAILED\\b.{0,2})|((\\d+)\\b SUCCESS\\b.{0,2}))+)`
Notification: `Unit Tests Results \\1`

This feature request is to allow the user to specify an output string to listen to and have an associated custom text notification display.

Start by locating the extension's terminal output handling and notification entry points; the issue does not name specific files. Done means users can specify an output pattern and associated notification text, with captured matches supported as shown by the regex example.

Written by the indexing model from the issue text.