[ffigen] Use `package:file` instead of `dart:io` directly

Currently FFIgen uses `dart:io` directly.

This forces our FFIgenPad prototype to use [IOOverrides](https://api.flutter.dev/flutter/dart-io/IOOverrides-class.html).

If we were to use `package:file`, we could potentially wire up all File reads and writes using a `FileSystem` that targets the emscripten file system when we use WASM.

Since this is a more general problem than just our use case, I've gone ahead and file an issue on the Dart SDK to see if others think that would be a good direction to go in:

* https://github.com/dart-lang/sdk/issues/56404

So let's wait on some feedback there before we head in this direction. (marking issue low prio)

Note, it would be tempting to run the the whole of FFIgen with a [MemoryFileSystem](https://pub.dev/documentation/file/latest/memory/MemoryFileSystem-class.html) and make our test suite blazing fast. However, that doesn't work due to libclang wanting to read from the `LocalFileSystem`.

FYI @TheComputerM @mannprerak2