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/string/literals.wren

@@ -1,5 +1,5 @@
-IO.print("".count)   // expect: 0
-IO.print("a string") // expect: a string
+System.print("".count)   // expect: 0
+System.print("a string") // expect: a string
 
 // Non-ASCII.
-IO.print("A~¶Þॐஃ") // expect: A~¶Þॐஃ
+System.print("A~¶Þॐஃ") // expect: A~¶Þॐஃ