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/map/count.wren

@@ -1,12 +1,12 @@
 var map = {}
-IO.print(map.count) // expect: 0
+System.print(map.count) // expect: 0
 map["one"] = "value"
-IO.print(map.count) // expect: 1
+System.print(map.count) // expect: 1
 map["two"] = "value"
-IO.print(map.count) // expect: 2
+System.print(map.count) // expect: 2
 map["three"] = "value"
-IO.print(map.count) // expect: 3
+System.print(map.count) // expect: 3
 
 // Adding existing key does not increase count.
 map["two"] = "new value"
-IO.print(map.count) // expect: 3
+System.print(map.count) // expect: 3