Transform > Shear: x/y axis constraint, not in transform modal

The Transform>Shear tool has the ability to constrain to the x or y axis, but doesn't use transform modal map like other transform tools do (move, rotate, scale). This means custom keys cannot be used to constrain Shear to x or y axis.

I realize its a bit different that the basic transform modes, but It would make sense to have it in the transform modal with other operators that can constrain along and axis.