builtin property: accept keyword arguments

this allows python code to use property(lambda:..., doc=...) idiom.

named versions for the fget, fset and fdel arguments are left out in the
interest of saving space; they are rarely used and easy to enable when
actually needed.

a test case is included.

py/qstrdefs.h

@@ -570,6 +570,7 @@ Q(property)
 Q(getter)
 Q(setter)
 Q(deleter)
+Q(doc)
 #endif
 
 #if MICROPY_PY_UZLIB