[editor] Add placeholder to empty text prompt input

# [editor] Add placeholder to empty text prompt input

Note the figma has the text "Type a prompt. (e.g. show me an astronaut riding a horse on the moon)", however that could be confusing for any model that doesn't have an image output. So, let's just use 'Type a prompt' for now

<img width="1360" alt="Screenshot 2023-12-29 at 8 14 38 PM" src="https://github.com/lastmile-ai/aiconfig/assets/5060851/a84d6ebb-77ae-4304-8689-0cdfa7236360">

python/src/aiconfig/editor/client/src/components/prompt/prompt_input/schema_renderer/PromptInputDataSchemaRenderer.tsx

@@ -20,6 +20,7 @@ export default memo(function PromptInputDataSchemaRenderer({
         <Textarea
           value={data ? (data as string) : ""}
           onChange={(e) => onChangeData(e.target.value)}
+          placeholder="Type a prompt"
         />
       );
     default:

python/src/aiconfig/editor/client/src/components/prompt/prompt_input/schema_renderer/PromptInputSchemaRenderer.tsx

@@ -71,6 +71,7 @@ export default memo(function PromptInputSchemaRenderer(props: Props) {
       <Textarea
         value={props.input as string}
         onChange={(e) => props.onChangeInput(e.target.value)}
+        placeholder="Type a prompt"
       />
     );
   } else {