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/string/escapes.wren

@@ -1,7 +1,7 @@
 // Escape characters.
-IO.print("\"")     // expect: "
-IO.print("\\")     // expect: \
-IO.print("(\n)")   // expect: (
+System.print("\"")     // expect: "
+System.print("\\")     // expect: \
+System.print("(\n)")   // expect: (
                    // expect: )
 
 // TODO: Non-printing escapes like \t.