Runtime: add new sed ftplugin (#12843)

diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 911de1e..237a805 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -207,6 +207,7 @@
 runtime/ftplugin/scala.vim		@derekwyatt
 runtime/ftplugin/scss.vim		@tpope
 runtime/ftplugin/sdoc.vim		@gpanders
+runtime/ftplugin/sed.vim		@dkearns
 runtime/ftplugin/sh.vim			@dkearns
 runtime/ftplugin/solution.vim		@dkearns
 runtime/ftplugin/spec.vim		@ignatenkobrain