Closes #5761

Useful for editing & re-triggering downstream chat events if necessary

Introduces:

• enter_pressed in ChatAreaInput because it's better than watching value change and parallels TextInput's enter_pressed

3. show_edit_icon for the new edit icon that toggles the ChatMessage between original object & ChatAreaInput; only shows if message is from user
4. edited for when an edit is submitted in ChatMessage
5. Feed watches messages for any edited event; if so, call edit_callback with contents, index, instance

def echo_callback(content, index, instance):
    return content

def edit_callback(content, index, instance):
    instance.objects[index + 1].object = content

feed = ChatFeed(edit_callback=edit_callback, callback=echo_callback, callback_user="Echo Guy")
feed.send(pn.widgets.TextInput(value="Hello World!"))
feed.show()