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/scheduler/add.wren

@@ -7,9 +7,9 @@ Scheduler.add {
 }
 
 // Does not run immediately.
-IO.print(run) // expect: false
+System.print(run) // expect: false
 
 Timer.sleep(0)
 
 // Runs when the main fiber suspends on IO.
-IO.print(run) // expect: true
+System.print(run) // expect: true