Make ReactLocalization the value of FluentContext

[stasm]

The value of FluentContext is now an instance of ReactLocalization which gained the parseMarkup method.

I also exposed FluentContext as part of the public API in case someone prefers to use it via FluentContext.Consumer or the useContext hook. In fact, I think we should declare these methods as preferrable to the withLocalization decorator, which suffers from not hoisting static members of the inner component, if any are present. #430 has a fix for it but perhaps we could live with this shortcoming of withLocalization if other methods are available.

[stasm]

@Gregoor Would you mind taking a look at this?

[Gregoor]

The changes look good to me 👍

[stasm]

Thanks :)