Automate Extension Publishing via GitHub Action

Currently, we publish our extensions by downloading the signed qtcli and running publish.ts. Instead of doing that, we should create a GitHub Action that handles publishing automatically.

An example command would be the following:

gh workflow run "Publish" --repo https://github.com/qt-labs/vscodeext -f release
# or for pre-release
gh workflow run "Publish" --repo https://github.com/qt-labs/vscodeext -f pre-release

The above command should publish both on the VSCode Marketplace and the Open VSX Registry.

Notes:

1. Only official developers should be allowed to trigger the action. Check access rights.

1. The action must run the same checks as publish.ts before publishing.

1. Tags should be created and pushed automatically.