Add CI script and token to support releasing to Marketplace

[gjsjohnmurray]

@daimor are you willing to take this? I guess you'll need help from @isc-rsingh to get the token in place, and to adjust settings on this repo. I don't expect frequent releases will be needed, since there's not much to fix or enhance in an extension pack. But having the CI in place seems like a good idea.

[daimor]

Yeah, sure

[gjsjohnmurray]

Thanks. And when working on the CI for this repo maybe see if you can make it handle the following release pathway.

1. Draft a release and save it as a draft.

2. Later, return to the draft release and use the "Publish release" button.

When I did this on the other repo yesterday step 2 didn't trigger the release workflow necessary to send a vsix to Marketplace.

[gjsjohnmurray]

@isc-rsingh please add the 3 secrets @daimor listed in PR #9 into this repo's Settings section.

Once those are in position I will create the first release of this extension pack.

[isc-rsingh]

Tokens added.

[gjsjohnmurray]

Thanks both. Now published.