[hxb] Add -main information

Hxb currently does not store `com.main_class` and `com.main` anywhere. It would be nice to (optionally?) support this so that we can have runnable archives without the need to specify `-main` again.

I'm not sure where this should go though because it's not part of any particular module's structure. We likely need some sort of manifest/bootstrap file in the generated .zip which we check by convention. Notably, this should still be in binary format because `com.main` is a `texpr option`.

Somewhat related, it would also be nice if we could store the module order, i.e. the output of `Finalization.sort_types`. That way we don't have to do this on every compilation.

Start by tracing how hxb generates the .zip archive and how Finalization.sort_types produces module order. Investigate where com.main_class and com.main could be represented in a binary manifest or bootstrap file by convention. Done means the archive can preserve the optional main information and module order without requiring -main or repeating compilation sorting.

Written by the indexing model from the issue text.