[skip ci] Add EXTENSIONS for @youkidearitai in mbstring (#15874)

Author: youkidearitai

EXTENSIONS

@@ -364,6 +364,7 @@ EXTENSION:           mbstring
 PRIMARY MAINTAINER:  Rui Hirokawa <hirokawa@php.net> (2001 - 2013)
                      Nikita Popov <nikic@php.net> (2017 - 2020)
                      Alex Dowad <alexinbeijing@gmail.com> (2021 - 2024)
+                     Yuya Hamada <youkidearitai@gmail.com> (2024 - 2024)
 MAINTENANCE:         Maintained
 STATUS:              Working
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