js: use osd-dimentions for mp.get_osd_{size,margins}

This matches lua's 11b9315b but with the lagacy field names which the
js code used previously.

Currently the property always returns an object (with dummy/last/null
field values if there are no dimensions), but the code is ready for
a future case where it might return null if there are no dimensions - at
which case it will forward the null, breaking backward compatibility for
a better API.