Add span tracking to allow emitting diagnostics everywhere

[oli-obk]

Instead of just tracking line numbers manually, this makes sure that everything preserves spans.

This is done by replicating most of the str API on Spanned<&str>, so that you can just use trim, to_string, chars... and keep span information at all times. I was not able to measure a performance impact on just check.

More can be done here, but I wanted to get a vibeck first before putting in more work.

Many situations already produce errors like

point of contact must be a single github username (found src/2025h2/TEMPLATE.md:16:22: must be a single Github username like @ghost)

where before they did not have file, line and col information

[lqd]

This looks like a good improvement to me. The error messages can be confusing rn, especially with the template bugs causing more of them.

#312 simplifies the project structure a lot, so it'd be cool if we can merge this PR after 312 🙏

[oli-obk]

Yea no worries. I'll happily rebase after that