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/number/is_nan.wren

@@ -1,5 +1,5 @@
-IO.print(1.isNan)      // expect: false
-IO.print((0/0).isNan)  // expect: true
+System.print(1.isNan)      // expect: false
+System.print((0/0).isNan)  // expect: true
 
 // Infinity is not NaN.
-IO.print((1/0).isNan)  // expect: false
+System.print((1/0).isNan)  // expect: false