Add shellcheck feature to the index (devcontainers#85)

Author: lukewiwa

_data/collection-index.yml

@@ -118,3 +118,8 @@
   contact: https://github.com/rjfmachado/devcontainer-features/issues
   repository: https://github.com/rjfmachado/devcontainer-features
   ociReference: ghcr.io/rjfmachado/devcontainer-features
+- name: Shellcheck
+  maintainer: lukewiwa
+  contact: https://github.com/lukewiwa/features/issues
+  repository: https://github.com/lukewiwa/features
+  ociReference: ghcr.io/lukewiwa/features