codereview.cfg: declare issue repo

For golang/go#9273

Change-Id: Ib473b7d493a17a9c519d1ac6737a4afaaa2feff0
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/388757
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

codereview.cfg

@@ -0,0 +1 @@
+issuerepo: golang/vscode-go