Add inline placeholders

Description

Currently, we have a placeholder option for textarea, text, and number. However, applying this option doesn't have any effect on inline editing, it only shows in the sidebar.

To achieve something similar, you can apply a default prop or a render default value.

However, it would be beneficial to also show the placeholder inline in the editor only. This would make it easier to guide users without polluting the data payload.

Proposals

Show the placeholder prop inline when using contentEditable: true.

Considerations

• We should be careful with the placeholder font color. Applying a hardcoded grey might not be feasible in this case. It could be better to simply apply a lower opacity and inherit the font color that's currently being used in the document.