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/tan.wren

@@ -1,3 +1,3 @@
-IO.print(0.tan)             // expect: 0
-IO.print((Num.pi / 4).tan)  // expect: 1
-IO.print((-Num.pi / 4).tan) // expect: -1
+System.print(0.tan)             // expect: 0
+System.print((Num.pi / 4).tan)  // expect: 1
+System.print((-Num.pi / 4).tan) // expect: -1