Carbonized form editor

[pinussilvestrus]

Related to https://github.com/camunda/team-hto/issues/143
Depends on bpmn-io/form-js#557

• Demo: https://demo-theming--camunda-form-playground.netlify.app/carbon

Todo:

• Incorporate Carbon Styles: Sass style sheet bpmn-io/form-js#842
• Find a way to add layer (to properly adapt the correct color schemes)
• Adapt recent changes
• Update carbonized editor styles: https://www.figma.com/file/CiTSN1vNETjIAy6W9jUIFp/Form-Editor?node-id=905%3A139488&t=TjQHiGcJYXc43Fa6-0
• Consider making @carbon/elements a peer dependency

[pinussilvestrus]

We recently introduced the carbon-styles via SASS, which should make it easier to integrate the Carbon components here (bpmn-io/form-js#842). The measures to add styled-components in this PR are obsolete.

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