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/map.markdown

@@ -50,8 +50,8 @@ map, returns `null`.
 
     :::dart
     var map = {"george": "harrison", "ringo": "starr"}
-    IO.print(map["ringo"]) // "starr".
-    IO.print(map["pete"])  // "null".
+    System.print(map["ringo"]) // "starr".
+    System.print(map["pete"])  // "null".
 
 ### **[**key**]=**(value) operator