w_debug: New message handling for development

i'm using this locally to diagnose my contributions and i believe that 
using this in winetricks would be beneficial

Signed-off-by: Jacob Hrbek <kreyren@rixotstudio.cz>

Author: Jacob Hrbek

src/winetricks

@@ -262,6 +262,15 @@ w_warn()
     unset _W_timeout
 }
 
+# Output debug message to stdin if debug variable is non-zero
+w_debug()
+{
+    local w_message="$1"
+
+    # shellcheck disable=SC2154
+    [ -n "$W_DEBUG" ] && printf 'DEBUG: %s\n' "$w_message"
+}
+
 # Display warning message to stderr (since it is called inside redirected code)
 # And give gui user option to cancel (for when used in a loop)
 # If user cancels, exit status is 1