Generalized falling back show + mimetype

Right now we handle the fact that things can define show(::IO, mimetype, ::MyType), vis the MimeWriter
which we list just like any other library. Generally last.

I suggest that we should instead include in the registry a list of mime-types for each filetype.
and then we should have generic code that checks showable for each.
The code in the MimeWriter is basically all the same.
Since the full-full_fidelity IOContext should just be ignored by anything that doesn't know what to do with it;
and listing multiple would achieve the same as else-if that html has.

There is also an argument that these should be at the top of the priority list to try
This is because show is often defined by same library as defines the object.
As such it has the most informatiom about what it wants to do.
Futhermor, it will not normally require loading a dependency.

Start with src/mimesave.jl and the existing MimeWriter implementation. Review how the registry represents file types, then determine how MIME types and generic showable checks would fit there; done means the fallback behavior is generalized without requiring each MIME case to be listed separately.