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/variable/local_in_middle_of_block.wren

@@ -3,13 +3,13 @@ class Foo {
 
   bar {
     var a = "a"
-    IO.print(a) // expect: a
+    System.print(a) // expect: a
     var b = a + " b"
-    IO.print(b) // expect: a b
+    System.print(b) // expect: a b
     var c = a + " c"
-    IO.print(c) // expect: a c
+    System.print(c) // expect: a c
     var d = b + " d"
-    IO.print(d) // expect: a b d
+    System.print(d) // expect: a b d
   }
 }