`nsIFilePicker::show()` is removed in Firefox 60 in favor of `open()`,
which takes a callback (and apparently has been preferred for a long
time).
There's no point switching to that, so this module is a version of
nsIFilePicker with an async `show()` that returns a promise and some
XPCOM-isms replaced (e.g., string paths instead of nsIFile).
