Storage: use user's path to store user data (only relevant on native builds)

[endel]

I'm documenting this in case anybody needs this in the future. PRs are very welcome!

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When building for native targets, our Storage class currently stores user data (the auth token) in the current folder, which can lead to file permission issues.

Chris from OpenFL (@dimensionscape) has pointed the right direction on how OpenFL solved this (thank you Chris!), by using the user's path:

Please take a look at https://github.com/openfl/openfl/blob/develop/src/openfl/net/SharedObject.hx if you'd like to see how we addressed this.

We use a location dubbed the applicationStorageDirectory which is resolved through our native extension in Lime https://github.com/openfl/lime/blob/bb3b31b08d75c0b9aa2dc1f3ab5f4539737f4d57/src/lime/system/System.hx#L419