There are no "non-normal" files, so "normal" is not needed. We have
symlinks and conflicts, but they are not files, so I think just "file"
is unambiguous.
I left `testutils::write_normal_file()` because there it's used to
mean "not executable file" (there's also a `write_executable_file()`).
I left `working_copy::FileType::Normal` since renaming `Normal` there
to `File` would also suggest we should rename `FileType`, and I don't
know what would be a better name for that type.
This migrates the native backend from Protobuf to Thrift since
Google's Protobuf team does let us import jj into Google's monorepo if
it uses a third-party Protobuf library.
Since the native backend is not supported, I didn't write any
migration code for it.
We can't remove `lib/src/protos/store.proto` yet, because it's also
used by the Git backend (only the `predecessors` and `change_id`
fields).
