add gitpod configuration

[GitMensch]

creating an environment with VBISAM for online and local editing as well as running all tests

I suggest to additional do the following:

• possibly register for Gitpod, if not done already
• apply for "Open Source contribution" (effectively upgrading the gratis account to a professional one without costs)
• login and install the app here, see https://github.com/apps/gitpod-io
• 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

[GitMensch]

more nice to use - as well as using git anywhere else - after #36