Expose the foldable area as well as angle

[kenchris]

There are use-cases for exposing the exact angle of a foldable hinge: https://github.com/w3c/screen-fold/issues/45#issuecomment-785845333

As some future devices are bound to have more than one foldable area and thus hinge, this spec seems to be the right place for it.

@anssiko @torgo @diekus @darktears

This was also discussed in the Second Screen Virtual meeting: w3c/secondscreen-wg#2