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-command/src/platform/is_terminal.rs

@@ -1,5 +1,4 @@
 use nu_engine::command_prelude::*;
-use nu_protocol::span;
 use std::io::IsTerminal as _;
 
 #[derive(Clone)]
@@ -57,12 +56,9 @@ impl Command for IsTerminal {
                 });
             }
             _ => {
-                let spans: Vec<_> = call.arguments.iter().map(|arg| arg.span()).collect();
-                let span = span(&spans);
-
                 return Err(ShellError::IncompatibleParametersSingle {
                     msg: "Only one stream may be checked".into(),
-                    span,
+                    span: Span::merge_many(call.arguments.iter().map(|arg| arg.span())),
                 });
             }
         };

crates/nu-command/src/platform/kill.rs

@@ -1,5 +1,4 @@
 use nu_engine::command_prelude::*;
-use nu_protocol::span;
 use std::process::{Command as CommandSys, Stdio};
 
 #[derive(Clone)]
@@ -96,7 +95,7 @@ impl Command for Kill {
                             })?
                             .span,
                         right_message: "signal".to_string(),
-                        right_span: span(&[
+                        right_span: Span::merge(
                             call.get_named_arg("signal")
                                 .ok_or_else(|| ShellError::GenericError {
                                     error: "Flag error".into(),
@@ -107,7 +106,7 @@ impl Command for Kill {
                                 })?
                                 .span,
                             signal_span,
-                        ]),
+                        ),
                     });
                 }
                 cmd.arg("-9");