[Github] Add pr-subscribers-infrastructure notifications (llvm#142697)

This patch sets up the new PRs labeller for a
pr-subscribers-infrastructure team that can be used for recieving
notifications about infrastructure changes in the monorepo. This is
primarily centered around the .ci directory currently where most of the
action is taking place.

Author: boomanaiden154

.github/new-prs-labeler.yml

@@ -1121,3 +1121,6 @@ tablegen:
   - llvm/include/TableGen/**
   - llvm/lib/TableGen/**
   - llvm/utils/TableGen/**
+
+infrastructure:
+  - .ci/**