Enable span properly to fix build failure with span feature disabled

[lo48576]

Summary

• Fix build error when span feature is disabled (due to --no-default-features (cargo CLI) or default-features = false (Cargo.toml)).
• Set error span correctly even when span feature is disabled.
  • This fixes v2_parser::failure_tests::bad_child_test failure with span feature disabled.

How to reproduce

1. Check out kdl-rs repository
   • main branch or the latest release (v6.3.3)
   • Older versions may also be affected, but I didn't check.
2. Run cargo check --no-default-features
3. You'll see build error.

$ cargo check --no-default-features
    Checking kdl v6.3.3 (/tmp/kdl2/kdl-rs)
error[E0308]: `if` and `else` have incompatible types
   --> src/v2_parser.rs:558:12
    |
543 |         let entry = if let Some(after_key) = after_key {
    |  ___________________-
544 | |           let (after_eq, value) = (
545 | |               node_space0.take(),
546 | |               cut_err(value.context(cx().lbl("property value"))),
...   |
549 | | /         value.map(|mut value| {
550 | | |             value.name = maybe_ident;
551 | | |             if let Some(fmt) = value.format_mut() {
552 | | |                 fmt.after_key = after_key.into();
...   | |
556 | | |             value
557 | | |         })
    | | |__________- expected because of this
558 | |       } else if let Some(ident) = maybe_ident {
    | |  ____________^
559 | | |         // It was ambiguous, but this ident is actually a value.
560 | | |         Some(KdlEntry {
561 | | |             format: Some(KdlEntryFormat {
...   | |
574 | | |             .flatten()
575 | | |     };
    | | |     ^
    | |_|_____|
    |   |_____`if` and `else` have incompatible types
    |         expected `Option<()>`, found `Option<KdlEntry>`
    |
    = note: expected enum `Option<()>`
               found enum `Option<entry::KdlEntry>`

warning: variable does not need to be mutable
   --> src/v2_parser.rs:751:34
    |
751 |                     ErrMode::Cut(mut pe) => {
    |                                  ----^^
    |                                  |
    |                                  help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` on by default

For more information about this error, try `rustc --explain E0308`.
warning: `kdl` (lib) generated 1 warning
error: could not compile `kdl` (lib) due to 1 previous error; 1 warning emitted
$