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/helper/init.zsh

@@ -29,3 +29,23 @@ function coalesce {
   done
   return 1
 }
+
+# is true on MacOS Darwin
+function is-darwin {
+  [[ "$OSTYPE" == darwin* ]]
+}
+
+# is true on Linux's
+function is-linux {
+  [[ "$OSTYPE" == linux* ]]
+}
+
+# is true on BSD's
+function is-bsd {
+  [[ "$OSTYPE" == *bsd* ]]
+}
+
+# is true on Cygwin (Windows)
+function is-cygwin {
+  [[ "$OSTYPE" == cygwin* ]]
+}