Librarification

[blyxxyz]

It would be nice to expose a library interface, to enable projects to build on top of xh along the lines of http-prompt. It would also help with #88.

This could be a pretty big change. We'd have to decide what to expose and commit to not breaking compatibility. Refactoring would become harder the more we expose.

Some concrete steps would be:

• Move most things out of main
• Change instances of pub to pub(crate)
• Add top-level re-exports
• (Maybe) add #[non_exhaustive] to certain public enums
• (Maybe) use custom error types instead of anyhow

It could help to have a consuming project that lives in the same repository (in a workspace).

At first glance, we might not have to expose that much, see how http-prompt calls it. But I haven't looked at other projects, and a richer API might be a lot more convenient.

Breaking the public interface after 1.0 would mean incrementing the major version number. I can think of a few approaches:

• Get this in place before a 1.0 release and be very careful to provide something we can remain compatible with.
• Be willing to increment the major version number even when nothing breaks for normal users of the tool.
• Move most of the code into a new package (e.g. xh_core) that can increment its major version freely, while the xh package becomes a wrapper around it with more conservative versioning.

[satvikpendem]

Coming from #289, to be honest I didn't realize you were already using reqwest underneath. I liked the single line of xh :8080 rather than the multiline usage of reqwest, but it's likely not enough of a big deal to need to librarify xh.