fix: allow getter-setter combinations in property declarations (#234)

The property_declaration rule previously used
choice(optional(getter), optional(setter)) which only allowed ONE
accessor. Properties with both getter and setter had the second
accessor spill out as a top-level declaration.

Change to optional(choice(seq(getter, optional(setter)),
seq(setter, optional(getter)))) to allow both accessors as children
of the property node, matching JetBrains PSI structure.

Author: VladimirMakaev