export builder types to public API

[OmarTawfik]

cc @oli-obk .. Thanks for the awesome crate!

The builder types were not exported out of the crate, which means users cannot construct new messages.
This PR exposes the builder types, hidden behind the feature flag.

Any feedback is appreciated.

[oli-obk]

Oh oops. Hmm... ideally that derive would produce something like MainType::builder() or similar so one can just use the type name instead of having to know about MainTypeBuilder... Do you happen to know if that is easily doable with some setting in the derive?

[OmarTawfik]

@oli-obk not sure if Type::builder() is supported in derive_builder. Is this something in the docs? I believe TypeBuilder is the expected output.

After changes in this PR, users are able to do this:

    let message = DiagnosticBuilder::default()
        .message("something is wrong".to_string())
        .rendered(None)
        .level(DiagnosticLevel::Error)
        .code(Some(diagnostic_code))
        .spans(vec![diagnostic_span])
        .children(vec![])
        .build()?;

[oli-obk]

I was just hoping that something more convenient would exist :)

Thanks for checking!