Support fixed position for prototypes
_I am..._

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| Requesting a new feature
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**Proposal/Feature-request:**

Would be great to support some kind of version of `position: fixed;` to be used in Sketch prototypes. Fixed position translates nicely to Sketch's `Fix position when scrolling`. This would allow for having bottom navigation tab bars, etc in prototypes.

React Native doesn't support `position: fixed;` though, so if the same component/styling is used in a React Native app, it may break it. Some alternative ideas that should solve that:

- Create a new `react-sketchapp` only styling prop: e.g. `prototypePosition: 'fixed'`
- Use `position: 'fixed'`, but document that this could be platform specific (`position: Platform.OS === 'sketch' ? 'fixed' : 'relative')
- Implement a new `ScrollView` component, and have an `Artboard` prop that makes anything not inside this component fixed. Not sure how much value this would have though for code re-use between web and React Native though when components like `FlatList` are more likely to be used.