Add prop for rendering a header above the list

[asdolo]

This PR adds a new optional prop that can be used to render a fixed header above the list.

Here's an example of what can be achieved:

<ModalSelector
  ...
  header={
    <View style={styles.header}>
      <Text variant="body-bold">What kind of facility is?</Text>
      <Text color="gray-4" variant="small">
        Please, select an option
      </Text>
    </View>
  }
/>

[mikaello]

Thanks for your contribution :-)

Could you:

• [x] add the example in this PR (with the styles) to the SampleApp
• [x] add the new prop to the TypeScript typings

?

[asdolo]

Done! 🙂 @mikaello

[mikaello]

Thanks, @asdolo !

Published to NPM as version 2.1.0:

https://www.npmjs.com/package/react-native-modal-selector/v/2.1.0