types(reactivity): fix corner case on UnwrapRef

[yoyo930021]

We need special string or number types sometime.
Ex. RelativePath, AbsolutePath

type AbsolutePath = string & { __brand: unknown }
function readFile (path: AbsolutePath) {
  // do some thing
}
function getAbsolutePath (path: string) {
  if (path.startWith('file://')) return path as AbsolutePath
}

I use it for safe use translation keys,
but code is broken when pass UnwrapRef on vetur template interpolation.