Add properties panel extension example

[nikku]

Proposed Changes

On top of #33, this adds a properties panel extension example:

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Try out via:

cd properties-panel-extension
npm install
npm start

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### Checklist

To ensure you provided everything we need to look at your PR:

* [x] **Brief textual description** of the changes present
* [x] **Visual demo** attached
* [x] **Steps to try out** present, i.e. [using the `@bpmn-io/sr` tool](https://github.com/bpmn-io/sr)
* [x] Related issue linked via `Closes {LINK_TO_ISSUE}` or `Related to {LINK_TO_ISSUE}`

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[barmac]

Thanks!