Strip code to the left and right in diagnostics for long lines

[phansch]

Analog to rust-lang/rust#63402

My initial idea is to hook this up here, where we would call into some other place that does the margin calculation:

annotate-snippets-rs/src/formatter/mod.rs

Line 185 in 1183568

DisplaySourceLine::Content { text, .. } => Some(format!(" {}", text)),

But I guess we'll also have to update the range of DisplaySourceLine::Annotation, so maybe it should happen earlier?

[oli-obk]

As a first step, one could move the rustc-independent code from rust-lang/rust#63402 to annotate-snippets and make rustc use it. This way we don't duplicate code.

[zzau13]

@phansch The only thing I do not know how to transfer to the library is annotated_file.multiline_depth.
I don't know exactly what it's for, I just see that it reduces the size of the column. I don't know exactly what it's for, I just see that it reduces the size of the column. At the moment I have adapted the margin and put an entry for FormatOptions {strip_code_line: Option <usize>, ..}
I will do the PR without treating this variable.