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/python/init.zsh

@@ -22,7 +22,7 @@ elif (( $+commands[pyenv] )); then
 else
   if [[ -n "$PYTHONUSERBASE" ]]; then
     path=($PYTHONUSERBASE/bin $path)
-  elif [[ "$OSTYPE" == darwin* ]]; then
+  elif is-darwin; then
     path=($HOME/Library/Python/*/bin(N) $path)
   else
     # This is subject to change.