Narratable report handler produces bad output with labels including EOL or EOF

[Benjamin-L]

The graphical report handler supports spans that include the EOL (or, at least, it will once #215 is fixed). These are rendered like this:

Source: source\ntext\n here
Span: (7, 5)

Error: oops::my::bad

  × oops!
   ╭─[bad_file.rs:1:1]
 1 │ source
 2 │ text
   · ──┬──
   ·   ╰── this bit here
 3 │   here
   ╰────
  help: try doing it better next time?

Passed the same input, the narratable handler produces:

Error: oops!
    Diagnostic severity: error
Begin snippet for bad_file.rs starting at line 1, column 1

snippet line 1: source
snippet line 2: text
    label starting at line 2, column 1: this bit here
snippet line 3:   here
    label ending at line 3, column 0: this bit here
diagnostic help: try doing it better next time?
diagnostic code: oops::my::bad

When the span extends past EOF, the result is:

Source: source\ntext
Span: (7, 5)

Error: oops!
    Diagnostic severity: error
Begin snippet for bad_file.rs starting at line 1, column 1

snippet line 1: source
snippet line 2: text
    label starting at line 2, column 1: this bit here
diagnostic help: try doing it better next time?
diagnostic code: oops::my::bad

With a trailing newline on the source input:

Source: source\ntext\n
Span: (7, 5)

Error: oops!
    Diagnostic severity: error
Begin snippet for bad_file.rs starting at line 1, column 1

snippet line 1: source
snippet line 2: text
    label at line 2, columns 1 to 4: this bit here
diagnostic help: try doing it better next time?
diagnostic code: oops::my::bad

---

All three of these cases should probably be rendered as label at line 2, columns 1 to 5: this bit here. When solving this, we should consider the edge cases from #215 (comment) as well.