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_fields.wren

@@ -7,8 +7,8 @@ class Foo {
   }
 
   fooPrint {
-    IO.print(_field1)
-    IO.print(_field2)
+    System.print(_field1)
+    System.print(_field2)
   }
 }
 
@@ -21,8 +21,8 @@ class Bar is Foo {
   }
 
   barPrint {
-    IO.print(_field1)
-    IO.print(_field2)
+    System.print(_field1)
+    System.print(_field2)
   }
 }