Add mdbook-i18n-normalize

This binary will take an old PO file with existing translations as
input. It runs `extract_messages` on every message, matches up the new
messages, and outputs a normalized PO file. The normalized file
maintains the old translations when possible.

How is this tested? I used the Korean translation of Comprehensive
Rust as a starting point since it is the most complete translation. I
ran the following commands

    MDBOOK_OUTPUT='{"markdown": {}}' MDBOOK_BOOK__LANGUAGE=ko mdbook build -d markdown-ko

to generate Markdown using the current translation.

I then ran

    mdbook-i18n-normalize po/ko.po po/ko-norm.po
    MDBOOK_OUTPUT='{"markdown": {}}' MDBOOK_BOOK__LANGUAGE=ko-norm mdbook build -d markdown-ko-norm

to generate Markdown using the normalized file. Before comparing the
two directories, I made all links inline links in the old
translation (`comrak` from https://github.com/kivikakk/comrak happens
to do this) and I formatted all files as well:

    for f in markdown-*/**/*.md; do
        comrak $f --to commonmark --gfm > $f.tmp && mv $f.tmp $f
        sed -i 's/<!-- end list -->//g' $f
    done
    dprint fmt 'markdown-ko-*/**/*.md'

After all this, `diff markdown-ko markdown-ko-norm` shows differences
in 9 files. Some of the differences are changes were we now invalidate
less of the file when the original English text changes. An example
can be seen in “Running the Course” which has a large list. We now
translate list items one-by-one:

```patch
diff --color --unified markdown-ko/running-the-course.md markdown-ko-norm/running-the-course.md
--- markdown-ko/running-the-course.md	2023-08-13 16:34:58.367680394 +0200
+++ markdown-ko-norm/running-the-course.md	2023-08-13 16:34:58.371680346 +0200
@@ -1,4 +1,4 @@
-# 강의 진행 방식
+# 강의 진행

 > 강사를 위한 안내 페이지입니다.

@@ -6,11 +6,10 @@

 강의를 실행하기 위한 준비:

-1. Make yourself familiar with the course material. We've included speaker notes
-   to help highlight the key points (please help us by contributing more speaker
-   notes\!). When presenting, you should make sure to open the speaker notes in
-   a popup (click the link with a little arrow next to "Speaker Notes"). This
-   way you have a clean screen to present to the class.
+1. 강의 자료를 숙지합니다. 주요 요점을 강조하기 위해 강의 참조 노트를
+   포함하였습니다. (추가적인 노트를 작성하여 제공해 주시면 감사하겠습니다.) 강의
+   참조 노트의 링크를 누르면 별도의 팝업으로 분리가 되며, 메인 화면에서는
+   사라집니다. 깔끔한 화면으로 강의를 진행할 수 있습니다.

 2. Decide on the dates. Since the course takes at least three full days, we
    recommend that you schedule the days over two weeks. Course participants have
```

This new fine-grained extraction technique allow us to use the
translation for the first list item, even though something has changed
for the later items.

Fixes #33.