feat: expose useContextMap

[hansottowirtz]

Exposes useAllContexts.

This would be useful for me in @bubblydoo/angular-react, as I can use this function to inject React contexts into Angular.

See the usage of useAllContexts (I've temporarily forked its-fine) and an example of an Angular component using it and the deployed example.

[CodyJasonBennett]

I'm debating between this and having useContextBridge accept a context map to write into. Not sure how confusing in/out arguments are though.

[hansottowirtz]

Maybe we can also expose a useContextBridgeFromContexts that accepts the map?

And then

export function useContextBridge() {
  const contexts = useAllContexts()
  return useContextBridgeFromContexts(contexts)
}

Might make the api feel bloated though.

[CodyJasonBennett]

I think I'm happy with this PR and exposing the traversal mechanism, but maybe name it useContextMap and export type ContextMap<T = any> = Map<React.Context<T>, T> as its return type.

[hansottowirtz]

I'm not sure if that is the correct type, as that would imply all the possible contexts on the context map are of type T.

Unfortunately the .get method of a Map is not generic itself, because this function would make sense:

function get<K>(key: React.Context<K>): K {
  return map.get(key)
}

We could rewrite the get and keys functions in this way and expose those instead of the map to make it more type-safe, but that would probably overcomplicate it. I suggest we just keep ContextMap = Map<React.Context<any>, any>.

[hansottowirtz]

Just had an idea, this also works and is not too complicated.

type ContextMap = Map<React.Context<any>, any> & {
  get<T>(context: React.Context<T>): T | undefined
}

[CodyJasonBennett]

I see. I'm fine with whatever, just that it be exported as a type.

[hansottowirtz]

Alright, then I think it can be merged.