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/core/fiber/transfer.wren

@@ -1,20 +1,20 @@
 var a = Fiber.new {
-  IO.print("a")
+  System.print("a")
 }
 
 var b = Fiber.new {
-  IO.print("b before")
+  System.print("b before")
   a.transfer()
-  IO.print("b after")
+  System.print("b after")
 }
 
 var c = Fiber.new {
-  IO.print("c before")
+  System.print("c before")
   b.transfer()
-  IO.print("c after")
+  System.print("c after")
 }
 
-IO.print("start") // expect: start
+System.print("start") // expect: start
 
 c.transfer()
 // expect: c before