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/call_transferred.wren

@@ -1,12 +1,12 @@
 var main = Fiber.current
 
 var fiber = Fiber.new {
-  IO.print("transferred")
-  IO.print(main.transfer())
-  IO.print("called")
+  System.print("transferred")
+  System.print(main.transfer())
+  System.print("called")
 }
 
 fiber.transfer() // expect: transferred
-IO.print("main") // expect: main
+System.print("main") // expect: main
 fiber.call()     // expect: null
                  // expect: called