- Clearly separates out and documents the variables that are most likely
  to need to change if copying the example app.
- Better differentiate between paths that are relative to the example
  and paths that are relative to the FDE repository, since apps using
  FDE would not be expected to be in the same directory the way the
  example is.
- Add a minimal "release" mode that enables that flag to disable Dart
  asserts, as on macOS and Windows.
- Remove the output directory override that puts the example app in the
  same output directory as the GN build (when building with GN).
  Combining them makes it less clear what's specific to the example,
  and what's part of building the library itself.