BCR reviewer: Add `diff_module` action type (#2158)

This action will diff new module versions in a BCR PR against its
previous version. Making it much easier to review changes between module
versions.

Test runs:
-
https://github.com/meteorcloudy/bazel-central-registry/actions/runs/12792158335/job/35663185312?pr=21
-
https://github.com/meteorcloudy/bazel-central-registry/actions/runs/12792705316/job/35663784300?pr=21
1 file changed