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/mixins/alert_error.pug

@@ -0,0 +1,2 @@
+mixin alert_error(message)
+  +alert(message)(class="alert-error")