Create a Gazelle plugin that adds the appropriate `all_files` `filegroup` declarations

When adding new Bazel packages to a workspace, it would be very useful to have the `all_files` `filegroup` added to the new packages and have the filegroup at the root updated with those targets.

Also, there could be a performance benefit to not use `glob()` and list the files.

Start by locating the repository's Gazelle integration and the existing all_files filegroup declarations for new packages and the root package. Determine how a plugin would add explicit file lists instead of glob(), then verify that newly added Bazel packages and the root filegroup receive the appropriate targets.

Written by the indexing model from the issue text.