Add activestorage.rbi.

Various methods were not being handled correctly (e.g. `variant?`) for
Rails models that were using ActiveStorage. This fixes many of those
issues.

rbi/annotations/activestorage.rbi

@@ -0,0 +1,15 @@
+# typed: strict
+
+class ActiveStorage::Attached::One
+  # @shim: Methods on Attached::One are delegated to `ActiveStorage::Blob` through `ActiveStorage::Attachment` using `method_missing`
+  include ActiveStorage::Blob::Analyzable
+  include ActiveStorage::Blob::Identifiable
+  include ActiveStorage::Blob::Representable
+end
+
+class ActiveStorage::Attached::Many
+  # @shim: Methods on Attached::Many are delegated to `ActiveStorage::Blob` through `ActiveStorage::Attachment` using `method_missing`
+  include ActiveStorage::Blob::Analyzable
+  include ActiveStorage::Blob::Identifiable
+  include ActiveStorage::Blob::Representable
+end