Use fixed column name for query xml output (#16461)

This is a breaking change.

Refs
https://discord.com/channels/601130461678272522/615329862395101194/1406413134402551869
Depends on (and includes) nushell/nushell#16459, which should be merged
first

Partially addresses nushell/nushell#15717

## Motivation and technical details

Variable column names are hard to work with. What's worse, currently
Nushell truncates the column name, making it even more unpredictable.
The only reliable way to deal with that is to rename the results column
like this:

```nushell
.... | query xml "//item/title|//title" | rename string_value | ...
```

so why would we make our users jump through these hoops? To provide some
parallels, `query db` does not add the query into its output, and
neither does `get`.

The choice of the column name comes from XPath spec:
https://www.w3.org/TR/xpath-10/#dt-string-value, adapted to Nushell
conventions

## Release notes summary - What our users need to know

`query xml` outputs the results (for nodeset results) in a column with a
fixed name.

Before:

```nushell 
open -r tests/fixtures/formats/jt.xml 
| query xml '//channel/title|//item/title'
# => ╭───┬───────────────────────────────────────────╮
# => │ # │           //channel/title|/...            │
# => ├───┼───────────────────────────────────────────┤
# => │ 0 │ JT                                        │
# => │ 1 │ Creating crossplatform Rust terminal apps │
# => ╰───┴───────────────────────────────────────────╯
```

Now:

```nushell 
open -r tests/fixtures/formats/jt.xml 
| query xml '//channel/title|//item/title'
# => ╭───┬───────────────────────────────────────────╮
# => │ # │             string_value                  │
# => ├───┼───────────────────────────────────────────┤
# => │ 0 │ JT                                        │
# => │ 1 │ Creating crossplatform Rust terminal apps │
# => ╰───┴───────────────────────────────────────────╯
```

## Tasks after submitting
<!-- Remove any tasks which aren't relevant for your PR, or add your own
-->
- [ ] Update the
[documentation](https://github.com/nushell/nushell.github.io)

crates/nu_plugin_query/src/query_xml.rs

@@ -89,19 +89,10 @@ pub fn execute_xpath_query(
         Ok(sxd_xpath::Value::Number(n)) => Ok(Value::float(n, call.head)),
         Ok(sxd_xpath::Value::String(s)) => Ok(Value::string(s, call.head)),
         Ok(sxd_xpath::Value::Nodeset(ns)) => {
-            // Some xpath statements can be long, so let's truncate it with ellipsis
-            let mut key = query_string.clone();
-            if query_string.len() >= 20 {
-                key.truncate(17);
-                key += "...";
-            } else {
-                key = query_string.to_string();
-            };
-
             let mut records: Vec<Value> = vec![];
             for n in ns.document_order() {
                 records.push(Value::record(
-                    record! {key.clone() => Value::string(n.string_value(), call.head)},
+                    record! {"string_value" => Value::string(n.string_value(), call.head)},
                     call.head,
                 ));
             }
@@ -205,7 +196,7 @@ mod tests {
 
         let expected = Value::list(
             vec![Value::test_record(record! {
-                "//qp:b/text()" => Value::string("yay", Span::test_data()),
+                "string_value" => Value::string("yay", Span::test_data()),
             })],
             Span::test_data(),
         );
@@ -293,7 +284,7 @@ mod tests {
 
         let expected = Value::list(
             vec![Value::test_record(record! {
-                "/elt" => Value::string("hello", Span::test_data()),
+                "string_value" => Value::string("hello", Span::test_data()),
             })],
             Span::test_data(),
         );