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

test/language/inheritance/inherit_methods.wren

@@ -1,16 +1,16 @@
 class Foo {
-  methodOnFoo { IO.print("foo") }
-  method(a) { IO.print("foo") }
-  method(a, b, c) { IO.print("foo") }
-  override { IO.print("foo") }
+  methodOnFoo { System.print("foo") }
+  method(a) { System.print("foo") }
+  method(a, b, c) { System.print("foo") }
+  override { System.print("foo") }
 }
 
 class Bar is Foo {
   construct new() {}
-  methodOnBar { IO.print("bar") }
-  method(a, b) { IO.print("bar") }
-  method(a, b, c, d) { IO.print("bar") }
-  override { IO.print("bar") }
+  methodOnBar { System.print("bar") }
+  method(a, b) { System.print("bar") }
+  method(a, b, c, d) { System.print("bar") }
+  override { System.print("bar") }
 }
 
 var bar = Bar.new()