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/range/where.wren

@@ -1,6 +1,6 @@
 var a = 1..3
 var b = a.where {|x| x > 1 }.toList
-IO.print(b) // expect: [2, 3]
+System.print(b) // expect: [2, 3]
 
 var c = a.where {|x| x > 10 }.toList
-IO.print(c) // expect: []
+System.print(c) // expect: []