Add pub to make_handler

[Palladinium]

I would like to set stable_eyre's hook as my eyre hook, but also do something else application-specific alongside it.

Currently, stable_eyre::install() just calls HookBuilder::install(), which in turn calls eyre::set_hook() like this:

stable-eyre/src/lib.rs

Line 168 in 48f50e3

crate::eyre::set_hook(Box::new(move |e| Box::new(self.make_handler(e))))?;

I'd like to be able to write something like this:

let hook_builder = stable_eyre::HookBuilder::default();
stable_eyre::eyre::set_hook(Box::new(move |e| {
    do_something_else(e);
    Box::new(hook_builder.make_handler(e))
}))?;

However, HookBuilder::make_handler() is private, and so I can't construct or wrap stable-eyre's Handler, and install() doesn't give any options to run extra code at hook-time.

This PR fixes that.

[ten3roberts]

Thank you.

Do you mind adding a short doc-comment to the function now that it is a public api?

[pksunkara]

@ten3roberts Can you please resolve this PR before I pull this repo into monorepo?

[ten3roberts]

Sure thing, will get to it when I'm back tonight