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/assignment/associativity.wren

@@ -4,6 +4,6 @@ var c = "c"
 
 // Assignment is right-associative.
 a = b = c
-IO.print(a) // expect: c
-IO.print(b) // expect: c
-IO.print(c) // expect: c
+System.print(a) // expect: c
+System.print(b) // expect: c
+System.print(c) // expect: c