feat: add getters for history util values

[lwhiteley]

canUndo, canRedo and getCurrentChangeDate do not cause a component to re-render when used by themselves.

To resolve this, equivalent getters are exposed to directly access the intended values

closes #10

[dai-shi]

The code looks okay with this. I didn't expect the use case of object getters and ref. It's nice to know.

Maybe we should prefer using this to proxyObject in all the places in the library code. (My original idea was to avoid this completely.)

[lwhiteley]

Maybe we should prefer using this to proxyObject in all the places in the library code. (My original idea was to avoid this completely.)

K will check in another changeset