refactor: extract IO class into separate module with wrenLoadIOLibrary and wren_io.c

Replace monolithic corelib.wren with per-class builtin/*.wren files and generate_builtins.py script. Move IO class definition and its native writeString_ method from wren_core.c to new wren_io.c, adding wrenGetArgumentString and wrenDefineStaticMethod to the public API. Update Makefile target from corelib to builtin and remove generate_corelib.py.

builtin/io.wren

@@ -0,0 +1,12 @@
+class IO {
+  static print(obj) {
+    IO.writeString_(obj.toString)
+    IO.writeString_("\n")
+    return obj
+  }
+
+  static write(obj) {
+    IO.writeString_(obj.toString)
+    return obj
+  }
+}