feat: add report formatter trait for custom reporter output

[zanieb]

Adds a ReportFormatter trait and a Reporter::report_with_formatter method allowing customization of formatting the report without reimplementing reporter internals. Currently, we use format_external and format_terms but we can refine formatting in the future as needed e.g. all the explain methods in the reporter.

Adds a DefaultStringReportFormatter which implements the existing format for the DefaultStringReporter.

You can see how much easier this is to use in the example diff at 725ec87

Some existing discussion at astral-sh#10

Addresses some of the concerns in #150

[Eh2406]

I don't think this trait pulls its weight, I think users would be just as well served by freestanding functions. Do you want to do that in this PR or leave it for follow-up?

[zanieb]

I agree this trait is a little weird unless users are expected to be passing around generic reporters. It makes a bit more sense now that there's another method, but I agree it seems sensible to remove. I'd prefer to track this in a new issue.

[Eh2406]

The current code only supports Output = String, eventually I prefer these functions return impl Display or take a &mut Write. Do you think Output pulls its weight? We could have the methods return String for now and fix it in a follow-up, or have the methods take &mut Write and return the bubbled up error. Thoughts?

[zanieb]

I think it makes sense to mirror the Reporter trait for now. We can tackle the rest in a subsequent refactor? I think that design deserves some dedicated consideration.

[mpizenberg]

I like it a lot too!

[Eh2406]

This is a huge step forward! There are more fixes to come, so let's merge this as is. Unless @mpizenberg have anything he wants done in this PR?

[mpizenberg]

Nothing to add, this is great!