internal/file: implement `ReadFileFS` for `VirtualFS` and `FileEntryFS`

### Operating System

- [ ] Windows
- [ ] macOS
- [ ] Linux
- [ ] FreeBSD
- [ ] OpenBSD
- [ ] Android
- [ ] iOS
- [ ] Nintendo Switch
- [ ] PlayStation 5
- [ ] Xbox
- [ ] Web Browsers

### What feature would you like to be added?

As the file system implements `ReadDirFS` (#3084), `ReadFileFS` should be implemented for consistency

### Why is this needed?

Just for consistency.