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/clear.wren

@@ -1,7 +1,7 @@
 var a = {1: 1, 2: 2, 3: 3}
 a.clear()
-IO.print(a)       // expect: {}
-IO.print(a.count) // expect: 0
+System.print(a)       // expect: {}
+System.print(a.count) // expect: 0
 
 // Returns null.
-IO.print({1: 2}.clear()) // expect: null
+System.print({1: 2}.clear()) // expect: null