Add GitHub Actions workflow file option

[badsyntax]

I was inspired by #165 to add a basic GitHub Actions workflow file.

This change adds a new option asking if a Github workflow file should be included.

The included workflow will test on all pushes, and publish on release (git tag), for the following types:

• ext-command-ts
• ext-command-js

The included workflow will just publish on release (git tag), for the following types:

• ext-colortheme
• ext-language
• ext-snippets
• ext-keymap
• ext-extensionpack
• ext-localization

The extension author then just needs to then add AZURE_TOKEN as a secret.

View an example workflow run here: https://github.com/badsyntax/vscode-generator-eslint-prettier-example/commit/83533b0082a00ffa91b91cd985f62739a5a2131a/checks?check_suite_id=349694101

[aeschli]

@joaomoreno You know more about CI scripts than me: If you could review, that would be great!

[joaomoreno]

Sorry @aeschli I really don't know much about GH Actions. This looks like a cool addition though!

[clarkio]

@joaomoreno @aeschli any interest in this still? It would be great to include scaffolding to help with steps to publish an extension.

I started down a path in creating my own template GitHub repository that includes pieces to cover that so I don't have to keep doing it manually. When I shared it more publicly it was suggested to look into including it in the generator which lead me to finding this PR :)