Added gitpod config

[jywarren]

Picking up where #483 left off:

Hi, I was pointed to your project and saw you were looking into adding gitpod support.
So here it is. I configured it so that it starts two terminals at start. The right one starts the live-server process and the left one is there to re-run the build (history up to get npm run build).
It also opens the preview on the examples directory at start.

You can test it on my fork, by clicking this link: https://gitpod.io/#https://github.com/svenefftinge/PublicLab.Editor

Fixes #475

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[jywarren]

Hmm. This needs a rebase, i guess, before it gets gitpodded?

[jywarren]

OK, i couldn't rebase this branch so i made my own: #558 Sorry to bounce around!