[wasm][debugger] Cancel Existing Single Step Request When Creating New (#2309)

[wasm][debugger] Cancel Existing Single Step Request When Creating New (#2309)

When stepping out of a c# breakpoint into the runtime (or JS... Not sure if that's accurate),
multiple single step requests can be created when the breakpoint is hit again.  In order to handle this scenario for wasm, we just cancel the existing one.