Stabilize Span::file and Span::local_file

[frankdavid]

Now that the corresponding methods in the stdlib have been stabilized as of Rust 1.88, it would be nice to also get access to these methods through stable proc-macro2.

[vidhanio]

This applies to many more of the previously unstable Span methods as well:

proc_macro::Span::line
proc_macro::Span::column
proc_macro::Span::start
proc_macro::Span::end
proc_macro::Span::file
proc_macro::Span::local_file

[frankdavid]

Could I get a thumbs-up, @dtolnay? I'm then happy to prepare the necessary PR.