add gitpod configuration

This config file helps for direct editing session on Gitpod without any further setup or manual build steps. This provides something similar to the gitab Web-IDE, but with an actual worker, so terminal, compile and tests all work, as well as any git command.

For people that want to try it, I suggest to do the following:

• possibly register for Gitpod, if not done already
• apply for "Open Source contribution" (effectively upgrading the no-cost/gratis account to a professional one without costs)
• later: configuring pre-builds

but as noted, that's "optional"; some benefits of Gitpod: nice performance, running without costs, full coding and building possible in the browser from anywhere, when pre-builds are active the development can start nearly in an instant, in theory also easy for people to start coding on the project.

If there are any suggestions on other steps / reasonable vscode addings, then I'm happy to include them - but the current definition is already quite useful.