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/while/syntax.wren

@@ -1,6 +1,6 @@
 // Single-expression body.
 var c = 0
-while (c < 3) IO.print(c = c + 1)
+while (c < 3) System.print(c = c + 1)
 // expect: 1
 // expect: 2
 // expect: 3
@@ -8,7 +8,7 @@ while (c < 3) IO.print(c = c + 1)
 // Block body.
 var a = 0
 while (a < 3) {
-  IO.print(a)
+  System.print(a)
   a = a + 1
 }
 // expect: 0