[react-jss] better typings for props passed to the inner component

[kof]

Currently classes is defined as optional, because if option inject is used, it might not get passed. We need to investigate for a way to type it based on inject prop and make sure user doesn't have to do defensive programming because of that.

injectSheet(styles, {inject: ['sheet', 'classes', 'theme']})

Same problem with ts typings.

[HenriBeck]

I don't think this is possible, at least with flow, might be doable with typescript though.

Is there a case for not injecting the classes prop though?

[kof]

Dunno, one could use sheet and not use classes, seemed logically that if I can customize one prop, why not customize all of them. But I see your point, it could be a solution for mainstream, where you always use classes

[HenriBeck]

I don't see any point where you would not want to inject the classes.
And sheet should only be injected when you need to access it which should rarely be.

An option which would make more sense to me is how the classes are merged.

[HenriBeck]

Done in #934