Restore `crate_universe/extension.bzl` shim with a deprecation warning (#4114)

#4005 removed `crate_universe/extension.bzl`, the deprecated re-export
of `crate_universe/extensions.bzl`, as part of dropping WORKSPACE
support.

Downstream projects may still load `crate` from that path via
`use_extension`, so any build that resolves `rules_rust` 0.71.0 with
such a module in its graph fails.
`protobuf` 35.1, the newest release on BCR, is one such module.

Restore the shim and print a deprecation warning at load time, giving
downstream modules a short window to migrate to `extensions.bzl`, e.g.:
- protocolbuffers/protobuf#28226.

Closes #4111.