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/number/scientific_literals.wren

@@ -1,9 +1,9 @@
 var x = 2.55e2
 
-IO.print(x) // expect: 255
-IO.print(x + 1) // expect: 256
-IO.print(x == 255) // expect: true
-IO.print(2.55e-2 is Num) // expect: true
-IO.print(x is Num) // expect: true
-IO.print(-2.55e2) // expect: -255
-IO.print(-25500e-2) // expect: -255
+System.print(x) // expect: 255
+System.print(x + 1) // expect: 256
+System.print(x == 255) // expect: true
+System.print(2.55e-2 is Num) // expect: true
+System.print(x is Num) // expect: true
+System.print(-2.55e2) // expect: -255
+System.print(-25500e-2) // expect: -255