Add MessageEditor component

[tobiasbueschel]

It would be great if AI Elements could also expose a MessageEditor component that would allow users to edit messages with the same / similar interface of our existing <PromptInput /> component so that one can e.g., delete file attachments or other context attachments from a previous message before sending it again.

Here is a reference to how a simple version (that doesn't match PromptInput) looks like in Chat SDK:

• https://github.com/vercel/ai-chatbot/blob/main/components/message-editor.tsx