Typecheck the TS files in pr-checks in verify-pr-checks job

Author: mbg

.github/workflows/script/verify-pr-checks.sh

@@ -31,3 +31,6 @@ if [ ! -z "$(git status --porcelain)" ]; then
     exit 1
 fi
 echo "Success: PR checks are up to date"
+
+# Typecheck the TS files
+npm exec tsc