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

doc/site/core/class.markdown

@@ -15,14 +15,14 @@ The superclass of this class.
     class Crustacean {}
     class Crab is Crustacean {}
 
-    IO.print(Crab.supertype)        // "Crustacean"
+    System.print(Crab.supertype) // "Crustacean".
 
 A class with no explicit superclass implicitly inherits Object:
 
     :::dart
-    IO.print(Crustacean.supertype)  // "Object"
+    System.print(Crustacean.supertype) // "Object".
 
 Object forms the root of the class hierarchy and has no supertype:
 
     :::dart
-    IO.print(Object.supertype)  // "null"
+    System.print(Object.supertype) // "null".