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/embedding-api.markdown

@@ -70,7 +70,7 @@ You can tell the VM to execute a string of Wren source code like so:
     :::c
     WrenInterpretResult result = wrenInterpret(vm,
         "<where>",
-        "IO.print(\"Hi!\")");
+        "System.print(\"Hi!\")");
 
 The first string parameter is a "source path". It's just an arbitrary string that describes where the source code is from. It's what shows up in stack traces if a runtime error occurs in the code. It can be whatever you want as long as it's not `NULL`.