fix: improve types

[songhn233]

While the files property can also be populated with new files, it is advised to add new files using only the addFile and addFiles methods.

When using React or other cases, such as a quickstart example:

const [files, setFiles] = useState<FilePondFile[]>([]);

<FilePond
  files={files}
  onupdatefiles={(files) => setFiles(files)}
/>

This will throw a type error because:

files?: Array<FilePondInitialFile | ActualFileObject | Blob | string>;
onupdatefiles?: ((files: FilePondFile[]) => void)

[CarelessCourage]

This is an issue we are dealing with too