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/this/nested_class.wren

@@ -2,16 +2,16 @@ class Outer {
   construct new() {}
 
   method {
-    IO.print(this) // expect: Outer
+    System.print(this) // expect: Outer
 
     Fn.new {
-      IO.print(this) // expect: Outer
+      System.print(this) // expect: Outer
 
       class Inner {
         construct new() {}
 
         method {
-          IO.print(this) // expect: Inner
+          System.print(this) // expect: Inner
         }
         toString { "Inner" }
       }