Error __DEV__ is not defined on Web if not patched manually in webpack

[AlanSl]

There are a couple of raw checks on __DEV__ in logger.ts that will cause a hard crash if @gorhom/portals is used in a default React Native Web build. The cause is that React Native Web doesn't define a global __DEV__ variable like React Native does.

Many libraries avoid this by doing a check that doesn't presume the existence of the global variable, e.g. if (typeof __DEV__ !== 'undefined' && __DEV__) instead of if (__DEV__).

Individual apps could work around having dependencies like this that do raw checks on __DEV__ by implementing their own webpack plugins to rewrite __DEV__ and then apply this to the appropriate modules in their build; but this can be a problem if using this library in a shared package like a components library, because it forces every consumer to add this config to their web builds, which can be difficult in some environments.

I'm happy to put up a PR for this [edit] - #30