What?

Support custom mobile Demo editor setup configuration.

Why?

Applying optional setup configuration files allows developers to modify
the Demo editor environment via Hooks, e.g. to set custom initialHtml
for the editor.

How?

Rely upon the experimental unstable_allowRequireContext Metro configuration to
enable searching for the optional setup-local.js configuration file via
require.context.

This approach is a workaround as loading the optional configuration file fails
when using allowOptionalDependencies, as outlined in facebook/metro#666 (comment).

Testing Instructions

Without custom configuration

1. Clone this branch.
2. Start the server via npm run native start:reset.
3. Build the app via npm run native ios or npm run native android.
4. Verify the Demo editor runs loading the existing initial-html
   configuration, displaying the expected blocks.

With custom configuration

/**
 * WordPress dependencies
 */
import { addFilter } from '@wordpress/hooks';

export default () => {
	addFilter(
		'native.block_editor_props',
		'core/react-native-editor',
		( props ) => {
			return {
				...props,
				initialHtml,
			};
		}
	);
};

const initialHtml = `
<!-- wp:heading -->
<h2 class="wp-block-heading" id="this-is-an-anchor">Just a Heading</h2>
<!-- /wp:heading -->
`;

1. Clone this branch.
2. Create a new file: packages/react-native-editor/src/setup-local.js
3. Populate the file with the "Example setup configuration" above.
4. Start the server via npm run native start:reset.
5. Build the app via npm run native ios or npm run native android.
6. Verify the Demo editor runs loading the local configuration file, displaying
   the blocks within the custom configuration.

Testing Instructions for Keyboard

n/a

Screenshots or screencast

n/a