Span ID Refactor - Step 1 (#12960)

# Description
First part of SpanID refactoring series. This PR adds a `SpanId` type
and a corresponding `span_id` field to `Expression`. Parser creating
expressions will now add them to an array in `StateWorkingSet`,
generates a corresponding ID and saves the ID to the Expression. The IDs
are not used anywhere yet.

For the rough overall plan, see
https://github.com/nushell/nushell/issues/12963.

# User-Facing Changes
Hopefully none. This is only a refactor of Nushell's internals that
shouldn't have visible side effects.

# Tests + Formatting

# After Submitting

crates/nu-command/src/system/run_external.rs

@@ -665,12 +665,7 @@ mod test {
     #[test]
     fn test_eval_argument() {
         fn expression(expr: Expr) -> Expression {
-            Expression {
-                expr,
-                span: Span::unknown(),
-                ty: Type::Any,
-                custom_completion: None,
-            }
+            Expression::new_unknown(expr, Span::unknown(), Type::Any)
         }
 
         fn eval(expr: Expr, spread: bool) -> Result<Vec<String>, ShellError> {