Explore removing `@mel.set` in < .. > Js.t

[Antonio Nuno Monteiro]

type rect = < height : int [@mel.set] > Js.t

and

foo##height #= ...

are probably not needed to have anymore.

[Javier Chávarri]

ftr mel.get is also supported with some payload:

melange/jscomp/test/class_setter_getter.ml

Line 29 in 7aa9b0b

 type yy2 = < height : int [@mel.get{undefined ; null}] [@mel.set] > Js.t