Add a GitHub Action to publish to GitHub releases, OpenVSX and the MS Marketplace

[filiptronicek]

This PR adds a GH action which can be executed to produce a new release. It consists of 4 jobs:

• package to prepare vsix file and upload it via the GitHub action
• publish to MS marketplace, one must configure a VSCE_PAT secret for this repo to make it work
• publish to OpenVSX, one must configure OVSX_PAT secret for this repo to make it work
• publish to GitHub releases, it will:
  • create a tag for a release to allow tracing from which commit the release was created
  • as well as publish a new GitHub release with corresponding vsix file Here is an example GH release: https://github.com/filiptronicek/NpmIntellisense/releases/tag/release%2F1.4.0 Runs of this workflow in the fork: https://github.com/filiptronicek/NpmIntellisense/blob/master/.github/workflows/publish.yml (It fails now because the publishing secrets are not configured).

Fixes #77

[ChristianKohler]

Thank you so much @filiptronicek and @akosyakov

I will have a look at it next week.