chore: add CODEOWNERS file for stdlib (#23519)

Until `3.8.0`, and because we are pinning the files for the scala 3 lib,
we add a `CODEOWNERS` so that GitHub can notify us in case a PR changes
the stdlib. This way, we can make sure that the PR doesn't go against
the spirit of pinning the files and that its intentions are actually
fulfilled.

[skip ci]

Author: hamzaremmal

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+# Any changes to the Scala 3 Standard Library must be approve
+# by one of the officers responsible of maintaining it
+/library/      @scala/stdlib-officers
+/library-aux/  @scala/stdlib-officers