feat(build): make global exports opt in

Default generated APIs to component-scoped modules and require global_exports(true) for legacy/global slot, handle, class, event, and lowering exports.

req: component/003

req: build/001

examples/v0/build.rs

@@ -1 +1,3 @@
-fn main() { slhx_build::app().run().unwrap(); }
+fn main() {
+    slhx_build::app().global_exports(true).run().unwrap();
+}