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/error-handling.markdown

@@ -108,7 +108,7 @@ For example, if you run this program:
     }
 
     var error = fiber.try()
-    IO.print("Caught error: ", error)
+    System.print("Caught error: " + error)
 
 It prints:
 
@@ -120,7 +120,7 @@ usual. When a fiber has been aborted because of a runtime error, you can also
 get the error from the fiber object. Continuing the above example:
 
     :::dart
-    IO.print(fiber.error)
+    System.print(fiber.error)
 
 This also prints: