Add content type metadata to config nu commands (#14666)

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# Description
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In v0.101.0 we got `config nu --default` and `config nu --doc` which
return a default config. That default config is valid `.nu`, so it
should have the metadata for it. We defined our MIME types [here in the
docs](https://www.nushell.sh/lang-guide/chapters/mime_types.html), so I
added that.

# User-Facing Changes
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Tools that read the metadata can now also detect that these two commands
are nushell scripts.

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- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

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crates/nu-command/src/env/config/config_nu.rs

@@ -1,4 +1,5 @@
 use nu_engine::command_prelude::*;
+use nu_protocol::PipelineMetadata;
 
 #[derive(Clone)]
 pub struct ConfigNu;
@@ -70,13 +71,21 @@ impl Command for ConfigNu {
         // `--default` flag handling
         if default_flag {
             let head = call.head;
-            return Ok(Value::string(nu_utils::get_default_config(), head).into_pipeline_data());
+            return Ok(Value::string(nu_utils::get_default_config(), head)
+                .into_pipeline_data_with_metadata(
+                    PipelineMetadata::default()
+                        .with_content_type("application/x-nuscript".to_string().into()),
+                ));
         }
 
         // `--doc` flag handling
         if doc_flag {
             let head = call.head;
-            return Ok(Value::string(nu_utils::get_doc_config(), head).into_pipeline_data());
+            return Ok(Value::string(nu_utils::get_doc_config(), head)
+                .into_pipeline_data_with_metadata(
+                    PipelineMetadata::default()
+                        .with_content_type("application/x-nuscript".to_string().into()),
+                ));
         }
 
         super::config_::start_editor("config-path", engine_state, stack, call)