Native Integration: 'dialog' API

[bryphe]

This issue is tracking the integration of a 'dialog' API for displaying native system dialogs for opening, saving files, alerting, etc.

As an inspiration, the Electron API would be a great start: https://electronjs.org/docs/api/dialog

This will require both creating Reason stubs and then using the C FFI to call the appropriate APIs for each platform.

[bryphe]

This lib looks like an interesting one to provide a consistent API for cross-platform file-picker dialogs: https://github.com/mlabbe/nativefiledialog

[OhadRau]

Do we need to pull in another dependency for this? It doesn't seem like an enormous amount of code, so we might just want to write our own API for this from scratch.