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/core.wren

@@ -0,0 +1,11 @@
+class List {
+  toString {
+    var result = "["
+    for (i in 0...this.count) {
+      if (i > 0) result = result + ", "
+      result = result + this[i].toString
+    }
+    result = result + "]"
+    return result
+  }
+}