Use subshell for open() to avoid job control output (#4366)

If it is not run as a sub-shell, using open and then subsequently closing the app will pollute the existing terminal with messages like:
  [1] 287915
  [1]+  Done  xdg-open "$@" > /dev/null 2>&1

Making it a subshell removes this noise and keeps it more like the open command in macOS.

default/bash/aliases

@@ -21,9 +21,9 @@ if command -v zoxide &> /dev/null; then
   }
 fi
 
-open() {
+open() (
   xdg-open "$@" >/dev/null 2>&1 &
-}
+)
 
 # Directories
 alias ..='cd ..'