Add async custom blocks feature

• Now is possible to add custom blocks when resolving a promise using the added insertAtomicBlockAsync API.
• ⚠️Deprecated value property. Use defaultValue instead.
• ⚠️Deprecated insertAtomicBlock reference function. Use insertAtomicBlockSync instead.
• Add example on how to upload an image within the editor.
• Add example on how to insert async atomic custom blocks.