Compatibility with MobX 6 useProxies: "ifavailable" option

[BATCOH]

Bug report

• I've checked documentation and searched for existing issues
• I've made sure my project is based on the latest MST version
• Fork this code sandbox or another minimal reproduction.

Sandbox link or minimal reproduction code
https://codesandbox.io/s/mobx-state-tree-todolist-ifavailable-t9w2z?file=/index.js

Describe the expected behavior
MST should work with useProxies: "ifavailable" option from mobx.

Describe the observed behavior
MST uses Object.defineProperty internally in addHiddenFinalProp/addHiddenWritableProp utils that lead to crash in ifavailable mode.
See also #2876@mobx
Since mobx@6.3.0 released with a convenient defineProperty from object-api, it seems to be a good idea to use it in MST.

I'm actually working on PR right now and will add it as soon as I can.

[jamonholmgren]

5.0.2 is released: https://github.com/mobxjs/mobx-state-tree/releases/tag/v5.0.2

Please test it on your project and report issues.

Thanks again @BATCOH !