[ci skip] Add Hamada-san to CODEOWNERS for mbstring

For the last couple years, Hamada-san has provided helpful feedback
on many GitHub issues and pull requests related to mbstring. He is in
a good position to help with reviewing future pull requests.

He agreed (via private mail) that it is fine to add his GitHub account
to CODEOWNERS.

Author: alexdowad

.github/CODEOWNERS

@@ -22,7 +22,7 @@
 /ext/intl @devnexen
 /ext/json @bukka
 /ext/libxml @nielsdos
-/ext/mbstring @alexdowad
+/ext/mbstring @alexdowad @youkidearitai
 /ext/mysqlnd @SakiTakamachi
 /ext/odbc @NattyNarwhal
 /ext/opcache @dstogov @iluuu1994