feat: replace IO class with System class and remove separate IO module

- Delete wren_io.c, wren_io.h, and io.wren source files
- Remove multi-arity print overloads and IO.read/IO.time methods
- Add System class with print, printAll, write, writeAll methods to core.wren
- Update all documentation examples and README to use System.print instead of IO.print

builtin/core.wren

@@ -213,3 +213,38 @@ class MapValueSequence is Sequence {
 }
 
 class Range is Sequence {}
+
+class System {
+  static print() {
+    writeString_("\n")
+  }
+
+  static print(obj) {
+    writeObject_(obj)
+    writeString_("\n")
+    return obj
+  }
+
+  static printAll(sequence) {
+    for (object in sequence) writeObject_(object)
+    writeString_("\n")
+  }
+
+  static write(obj) {
+    writeObject_(obj)
+    return obj
+  }
+
+  static writeAll(sequence) {
+    for (object in sequence) writeObject_(object)
+  }
+
+  static writeObject_(obj) {
+    var string = obj.toString
+    if (string is String) {
+      writeString_(string)
+    } else {
+      writeString_("[invalid toString]")
+    }
+  }
+}