Add Span merging functions (#12511)

# Description
This PR adds a few functions to `Span` for merging spans together:
- `Span::append`: merges two spans that are known to be in order.
- `Span::concat`: returns a span that encompasses all the spans in a
slice. The spans must be in order.
- `Span::merge`: merges two spans (no order necessary).
- `Span::merge_many`: merges an iterator of spans into a single span (no
order necessary).

These are meant to replace the free-standing `nu_protocol::span`
function.

The spans in a `LiteCommand` (the `parts`) should always be in order
based on the lite parser and lexer. So, the parser code sees the most
usage of `Span::append` and `Span::concat` where the order is known. In
other code areas, `Span::merge` and `Span::merge_many` are used since
the order between spans is often not known.

crates/nu_plugin_polars/src/dataframe/values/nu_dataframe/between_values.rs

@@ -1,7 +1,7 @@
 use super::{operations::Axis, NuDataFrame};
 use nu_protocol::{
     ast::{Boolean, Comparison, Math, Operator},
-    span, ShellError, Span, Spanned, Value,
+    ShellError, Span, Spanned, Value,
 };
 use num::Zero;
 use polars::prelude::{
@@ -17,9 +17,10 @@ pub(super) fn between_dataframes(
     right: &Value,
     rhs: &NuDataFrame,
 ) -> Result<NuDataFrame, ShellError> {
-    let operation_span = span(&[left.span(), right.span()]);
     match operator.item {
-        Operator::Math(Math::Plus) => lhs.append_df(rhs, Axis::Row, operation_span),
+        Operator::Math(Math::Plus) => {
+            lhs.append_df(rhs, Axis::Row, Span::merge(left.span(), right.span()))
+        }
         _ => Err(ShellError::OperatorMismatch {
             op_span: operator.span,
             lhs_ty: left.get_type().to_string(),
@@ -37,7 +38,7 @@ pub(super) fn compute_between_series(
     right: &Value,
     rhs: &Series,
 ) -> Result<NuDataFrame, ShellError> {
-    let operation_span = span(&[left.span(), right.span()]);
+    let operation_span = Span::merge(left.span(), right.span());
     match operator.item {
         Operator::Math(Math::Plus) => {
             let mut res = lhs + rhs;

crates/nu_plugin_polars/src/dataframe/values/utils.rs

@@ -1,4 +1,4 @@
-use nu_protocol::{span as span_join, ShellError, Span, Spanned, Value};
+use nu_protocol::{ShellError, Span, Spanned, Value};
 
 // Default value used when selecting rows from dataframe
 pub const DEFAULT_ROWS: usize = 5;
@@ -20,8 +20,8 @@ pub(crate) fn convert_columns(
             span: Some(span),
             help: None,
             inner: vec![],
-        })
-        .map(|v| v.span())?;
+        })?
+        .span();
 
     let res = columns
         .into_iter()
@@ -29,7 +29,7 @@ pub(crate) fn convert_columns(
             let span = value.span();
             match value {
                 Value::String { val, .. } => {
-                    col_span = span_join(&[col_span, span]);
+                    col_span = col_span.merge(span);
                     Ok(Spanned { item: val, span })
                 }
                 _ => Err(ShellError::GenericError {
@@ -70,7 +70,7 @@ pub(crate) fn convert_columns_string(
             let span = value.span();
             match value {
                 Value::String { val, .. } => {
-                    col_span = span_join(&[col_span, span]);
+                    col_span = col_span.merge(span);
                     Ok(val)
                 }
                 _ => Err(ShellError::GenericError {