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.

build.zig.zon

@@ -1,6 +1,6 @@
 .{
     .name = .pugz,
-    .version = "0.1.5",
+    .version = "0.1.6",
     .fingerprint = 0x822db0790e17621d, // Changing this has security and trust implications.
     .minimum_zig_version = "0.15.2",
     .dependencies = .{},