line terminators when:
a) running under windows, and
b) the file on the disk (when DrRacket first opens it)
contains all CRLF line endings.
In all other situations, the file is saved with LF terminators.
Before this commit, DrRacket would always use the platform-specific
convention, regardless of the original content of the file.
closes PR 12242
original commit: 4a2757f2004fa84901cbae7ff9a257616ce7acaa