`String#+@` is 2-3 times faster than `String#dup` because it can
directly go through `rb_str_dup` instead of using the generic
much slower `rb_obj_dup`.

This fact led to the existance of the ugly `Performance/UnfreezeString`
rubocop performance rule that encourage users to rewrite the much
more readable and convenient `"foo".dup` into the ugly `(+"foo")`.

Let's make that rubocop rule useless.

```
compare-ruby: ruby 3.3.0dev (2023-11-20T02:02:55Z master 701b0650) [arm64-darwin22]
last_commit=[ruby/prism] feat: add encoding for IBM865 (https://github.com/ruby/prism/pull/1884)
built-ruby: ruby 3.3.0dev (2023-11-20T12:51:45Z faster-str-lit-dup 6b745bbc5d) [arm64-darwin22]
warming up..

|       |compare-ruby|built-ruby|
|:------|-----------:|---------:|
|uplus  |     16.312M|   16.332M|
|       |           -|     1.00x|
|dup    |      5.912M|   16.329M|
|       |           -|     2.76x|
```