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/utility/init.zsh

@@ -134,9 +134,9 @@ if zstyle -t ':prezto:module:utility:grep' color; then
 fi
 
 # macOS Everywhere
-if [[ "$OSTYPE" == darwin* ]]; then
+if is-darwin; then
   alias o='open'
-elif [[ "$OSTYPE" == cygwin* ]]; then
+elif is-cygwin; then
   alias o='cygstart'
   alias pbcopy='tee > /dev/clipboard'
   alias pbpaste='cat /dev/clipboard'
@@ -166,7 +166,7 @@ fi
 alias df='df -kh'
 alias du='du -kh'
 
-if [[ "$OSTYPE" == (darwin*|*bsd*) ]]; then
+if is-darwin || is-bsd; then
   alias topc='top -o cpu'
   alias topm='top -o vsize'
 else