Fix mixin expansion in conditionals and include resolution in extends

- mixin.zig: expandNode and expandNodeWithArgs now recurse into
  node.consequent and node.alternate for Conditional nodes
- view_engine.zig: process includes and collect mixins from child
  template before extracting blocks in processExtends

This fixes mixin calls inside if/else blocks not being rendered
in compiled templates.

build.zig.zon

@@ -1,6 +1,6 @@
 .{
     .name = .pugz,
-    .version = "0.3.9",
+    .version = "0.3.10",
     .fingerprint = 0x822db0790e17621d, // Changing this has security and trust implications.
     .minimum_zig_version = "0.15.2",
     .dependencies = .{},