[meta] add GitPod configuration

[Decipher]

Is your feature request related to a problem? Please describe.
As a contributor I want GitPod integration so I can contribute to this project without having to setup an environment.

Describe the solution you'd like

• Add GitPod config
• Add pre-build tasks
  • Setup development environment
  • Install dependencies
  • ...
• Add VS code extensions

Additional context

• https://drupalpod.com
• https://www.gitpod.io/docs/config-gitpod-file
• https://github.com/druxt/druxt.js/blob/develop/.gitpod.yml