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/module/cyclic_import/a.wren

@@ -1,9 +1,9 @@
 // nontest
-IO.print("start a")
+System.print("start a")
 
 var A = "a value"
-IO.print("a defined ", A)
+System.print("a defined " + A)
 import "b" for B
-IO.print("a imported ", B)
+System.print("a imported " + B)
 
-IO.print("end a")
+System.print("end a")