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/limit/many_inherited_fields.wren

@@ -131,8 +131,8 @@ class Foo {
   }
 
   foo {
-    IO.print(_field1)
-    IO.print(_field128)
+    System.print(_field1)
+    System.print(_field128)
   }
 }
 
@@ -269,8 +269,8 @@ class Bar is Foo {
   }
 
   bar {
-    IO.print(_field129)
-    IO.print(_field255)
+    System.print(_field129)
+    System.print(_field255)
   }
 }