CI: Deploy to the VSCode marketplace

[jiribenes]

Resolves #21

Untested [EDIT: see below!], needs VSCE_PAT (a personal access token for publishing an extension, see here)

[jiribenes]

Could you generate a PAT for VSCode Marketplace (see link above) and add it to the repo secrets, @b-studios?
Then we could merge this and test it -- it's not a one-click release, but it's close enough for what we need.

[jiribenes]

Thanks for the token, let's try this! :)

[jiribenes]

Everything seems to have worked the first time 🥳
Since I can't put it into the README, here's how to release a new version:

---

Run the following commands to release a new version:

# Make sure you're on the latest `master`

git checkout master && git pull

# Depending on if the new change is minor or a patch (choose one)
# Creates a commit with the version bump and a tag

npm version [minor/patch] -m "Release v%s"

# Push the resulting commit

git push 

# Push the tag

git push --tags

then go see if the release went okay in the Actions tab: https://github.com/effekt-lang/effekt-vscode/actions/workflows/deploy.yml.

[b-studios]

Since I can't put it into the README

Can't we put this into something like CONTRIBUTE.md or the like?