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/core/system/print_all.wren

@@ -0,0 +1,8 @@
+class Foo {
+  construct new() {}
+
+  toString { "Foo" }
+}
+
+System.printAll([]) // expect:
+System.printAll([1, true, Foo.new(), "s"]) // expect: 1trueFoos