[Publishing] Send message to Slack channel on publish (success or failure)

[janpio]

https://github.com/prisma/vscode/blob/master/.github/workflows/publish.yml already automated publishing this extension, now we should also add something that notifies us. A Slack message should be a simple way to do that. (@steebchen already built that for something similar)

[janpio]

@steebchen can you link to some of the places where you implemented something like that so the person picking this up can see how this already works for us?

[steebchen]

GitHub Action workflow relevant lines:

https://github.com/prisma/prisma2-e2e-tests/blob/master/.github/workflows/test.yaml#L4-L18

This invokes this shell script: https://github.com/prisma/prisma2-e2e-tests/blob/master/.github/scripts/slack-workflow-status.sh
Which invokes this javascript file: https://github.com/prisma/prisma2-e2e-tests/blob/master/.github/slack/notify.js

(please also note that you need to have a package.json & lockfile in the js file directory https://github.com/prisma/prisma2-e2e-tests/tree/master/.github/slack and invoke yarn install to install these before executing the file)

I chose this over existing slack actions to be flexible. A simple existing GitHub community slack action might be enough depending on the use case.