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/sin.wren

@@ -1,6 +1,6 @@
-IO.print(0.sin)             // expect: 0
-IO.print((Num.pi / 2).sin)  // expect: 1
+System.print(0.sin)             // expect: 0
+System.print((Num.pi / 2).sin)  // expect: 1
 
 // these should of course be 0, but it's not that precise
-IO.print(Num.pi.sin)        // expect: 1.2246467991474e-16
-IO.print((2 * Num.pi).sin)  // expect: -2.4492935982947e-16
+System.print(Num.pi.sin)        // expect: 1.2246467991474e-16
+System.print((2 * Num.pi).sin)  // expect: -2.4492935982947e-16