feat(helper): add os-type helper functions

Add the following functions:

- is-darwin
- is-linux
- is-bsd
- is-cygwin

And apply them everywhere I found code doing that what these functions do.

modules/rsync/init.zsh

@@ -23,7 +23,7 @@ fi
 
 # macOS and HFS+ Enhancements
 # https://bombich.com/kb/ccc5/credits
-if [[ "$OSTYPE" == darwin* ]] && grep -q 'file-flags' <(rsync --help 2>&1); then
+if is-darwin && grep -q 'file-flags' <(rsync --help 2>&1); then
   _rsync_cmd="${_rsync_cmd} --crtimes --fileflags --protect-decmpfs --force-change"
 fi