Standardize on TW_ prefix for custom env vars

Author: japhb

lib/Terminal/Widgets/Simple/App.rakumod

@@ -72,7 +72,7 @@ class Terminal::Widgets::Simple::App is Terminal::Widgets::App {
         $*ERR.out-buffer = False;
 
         # Turn on debugging if requested in environment
-        $PROCESS::DEBUG //= +(%*ENV<TERMINAL_WIDGETS_DEBUG> // 0);
+        $PROCESS::DEBUG //= +(%*ENV<TW_DEBUG> // 0);
 
         # Provide hook for subclasses to perform boot-time initialization
         self.boot-init(|c);