Support --unicode-char:false with --standard-libraries

[robin-aws]

This would close the last gap between DafnyStandardLibraries and dafny-lang/libraries: bringing over UnicodeStringsWithoutUnicodeChar.dfy and customizing the externs for FileIO. It would mean duplicating the doo files for the two different modes. We would still only pre-translate the --unicode-char:true mode doo files into the Dafny runtime - see #5002 for how users can pre-translate the --unicode-char:false doo files themselves if necessary.

[keyboardDrummer]

I'm doubt we want to support this in the sense that we describe it as supported and document it. I think a hidden feature could be an option though.