feat(highlighting): implement syntax highlighting for unnamed source codes

[glevco]

Currently, both name and language are part of the SpanContents abstraction, which comes from the NamedSource struct. It would make more semantic sense if those were part of the SourceCode abstraction instead. This would allow us to use syntax highlighting in unnamed source codes, which is not currently possible. This PR proposes this.

Changes

• Rename NamedSource to MietteSourceCode, adding language to it.
• Move name() and language() from SpanContents trait to SourceCode trait.
• Move name and language from MietteSpanContents struct to MietteSourceCode struct.

This is a breaking change because it refactors public APIs.

Before

let src = NamedSource::new("bad_file", "fn hello() {}").with_language("Rust");
let report = miette!("oops!").with_source_code(src);

After

let src = MietteSourceCode::new("fn hello() {}").with_name("bad_file").with_language("Rust");
let report = miette!("oops!").with_source_code(src);

// unnamed with language, not possible before!
let src = MietteSourceCode::new("fn hello() {}").with_language("Rust");
let report = miette!("oops!").with_source_code(src);

[zkat]

I generally like this, but I'm gonna have to think more about moving those specific things around. I think there might be other things to be done about the struct to-be-formerly-known-as-NamedSource but I'll try and remember what else I thought might be nice.

I also don't know when I'll want to have a breaking release of miette. I try to keep them very very sparse, so if you can think of a way to do this without the breaking change, even if we eventually do the "nice" version, then that'll get unnamed source code highlighting working sooner for the community.

[glevco]

@zkat sure, I can do that. I'll simply create the MietteSourceCode like in this PR, but as a new struct instead of replacing the NamedSource with it. Then you can deprecate NamedSource and eventually remove it.