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/perl/init.zsh

@@ -37,7 +37,7 @@ fi
 # Local Module Installation
 #
 
-if [[ "$OSTYPE" == darwin* ]]; then
+if is-darwin; then
   # Perl is slow; cache its output.
   cache_file="${TMPDIR:-/tmp}/prezto-perl-cache.$UID.zsh"
   perl_path="$HOME/Library/Perl/5.12"