[libc++] Add myself to the credits.

Noticed while updating the credits for the to_chars patch.

GitOrigin-RevId: cb68fc814af63a7dc2f696fccbcc6971c76956bd
diff --git a/CREDITS.TXT b/CREDITS.TXT
index 597c5fc..fc442f4 100644
--- a/CREDITS.TXT
+++ b/CREDITS.TXT
@@ -149,6 +149,10 @@
 E: klaas at klaasgaaf dot nl
 D: Minor bug fix.
 
+N: Mark de Wever
+E: koraq at xs4all dot nl
+D: Format library support.
+
 N: Zhang Xiongpang
 E: zhangxiongpang@gmail.com
 D: Minor patches and bug fixes.