Added a beta release branch

[dalyIsaac]

This gives us the ability to have beta releases.

This is based on the unstable workflow.

To create a new release:

1. Create beta tag on your local branch
2. Push to the remote

The workflow builds workspacer and generates an installer. It deletes the beta tag and replaces it with the version number.

The version number is formatted like v0.9.11-beta.2. The final number in the versioning relies on the github.run_number - this means that it's possible to have v0.9.11-beta.2 followed by v0.9.12-beta.3.

The beta GitHub action is based on the unstable.yml workflow. I'm not entirely sure how to test this locally - hopefully if there's any errors they'll be picked up in review 🤞.

I've tested the workflow on https://github.com/dalyIsaac/workspacer-actions-test

[sitiom]

There is act but I haven't tried this out myself yet. I just usually run the actions on the fork I am working on. Maybe fork the project and run it there to see if it works fine.

[sitiom]

LGTM!