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.pug

@@ -0,0 +1,5 @@
+mixin alert(message)
+  div.alert(role="alert" class!=attributes.class)
+    svg(xmlns="http://www.w3.org/2000/svg" class="h-6 w-6 shrink-0 stroke-current" fill="none" viewBox="0 0 24 24")
+      path(stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10 14l2-2m0 0l2-2m-2 2l-2-2m2 2l2 2m7-2a9 9 0 11-18 0 9 9 0 0118 0z")
+    span= message