utils/file_reader library

Cross-platform file reading.

Picks the dart:io implementation on mobile/desktop and a stub that throws UnsupportedError on web — so the package compiles and runs on all platforms, including Flutter web (both JS and Wasm).

The condition keys off dart.library.io (native), defaulting to the web stub. Keying off dart.library.html would pull in dart:io on Wasm — where neither dart:html nor dart:io is available — breaking Wasm compilation.