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/timer/sleep_called_fibers.wren

@@ -1,21 +1,21 @@
 import "timer" for Timer
 
 var a = Fiber.new {
-  IO.print("a before")
+  System.print("a before")
   Timer.sleep(10)
-  IO.print("a after")
+  System.print("a after")
 }
 
 var b = Fiber.new {
-  IO.print("b before")
+  System.print("b before")
   a.call()
-  IO.print("b after")
+  System.print("b after")
 }
 
 // All fibers are suspended since they were directly called and not scheduled.
-IO.print("before")  // expect: before
+System.print("before")  // expect: before
 b.call()            // expect: b before
                     // expect: a before
                     // expect: a after
                     // expect: b after
-IO.print("done")    // expect: done
+System.print("done")    // expect: done