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/method/static.wren

@@ -7,7 +7,7 @@ class Foo {
   static bar(arg) { "on metaclass " + arg }
 }
 
-IO.print(Foo.new().bar)        // expect: on instance
-IO.print(Foo.bar)              // expect: on metaclass
-IO.print(Foo.new().bar("arg")) // expect: on instance arg
-IO.print(Foo.bar("arg"))       // expect: on metaclass arg
+System.print(Foo.new().bar)        // expect: on instance
+System.print(Foo.bar)              // expect: on metaclass
+System.print(Foo.new().bar("arg")) // expect: on instance arg
+System.print(Foo.bar("arg"))       // expect: on metaclass arg