Readd an export

Some stuff in web touches this private type. They're doing a naughty
thing but lets keep the status quo for now and we can chide them later

src/components/TextField/TextField.tsx

@@ -40,7 +40,7 @@ interface State {
   id: string;
 }
 
-interface BaseProps {
+export interface BaseProps {
   /** Text to display before value */
   prefix?: React.ReactNode;
   /** Text to display after value */