Add filter for customising the WordPress logo animation for preview

[noisysocks]

In #10896, we added an animated WordPress logo to indicate that the post preview was loading.

There's been repeated requests for the ability to customise this logo using e.g. a filter.

Such a filter could wrap around the .editor-post-preview-button__interstitial-message component, allowing developers to completely swap out the Generating preview… UI:

wp.hooks.addFilter( 'editor.interstitialMessage', function() {
    return wp.element.createElement( 'div', { className: 'my-custom-message' },
        wp.element.createElement( 'p', null, 'Post preview is being generated! 💃' )
    );
} );

[vs999]

Where should I put this code?
I tried in the functions.php of my theme but it broke my site.
thanks