fix: avoid variable shadowing in nested mixin calls with same parameter name

When a mixin calls another mixin passing a variable with the same name
as the parameter (e.g., +alert(message) where alert has param message),
skip generating redundant const declaration since the variable is already
in scope.

Also adds missing alert.pug mixin for demo project.

examples/demo/views/page-a.pug

@@ -24,3 +24,5 @@ block content
     br
     +input_text("lastName", "Last Name", "last name")
     submit sumit
+    if error
+      +alert_error(error)