Add metadata on open --raw with bytestreams (#14141)

# Description

This PR closes #14137 and allows the display hook to be set on byte
streams. So, with a hook like this below.
```nushell
display_output: {
    metadata access {|meta| match $meta.content_type? {
        "application/x-nuscript" | "application/x-nuon" | "text/x-nushell" => { nu-highlight },
        "application/json" => { ^bat --language=json --color=always --style=plain --paging=never },
        _ => {},
        }
    } | table
}
```
You could type `open toolkit.nu` and the text of toolkit.nu would be
highlighted by nu-highlight. This PR also changes the way content-type
is assigned with `open`. Previously it would only assign it if `--raw`
was specified.

Lastly, it changes the `is_external()` function to only say
`ByteStreamSource::Child`'s are external instead of both Child and
`ByteStreamSource::File`. Again, this was to allow the hook to function
properly. I'm not sure what negative ramifications changing
`is_external()` could have, but there may be some?

# User-Facing Changes
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helps us keep track of breaking changes. -->

# Tests + Formatting
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

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crates/nu-command/src/filesystem/open.rs

@@ -146,6 +146,9 @@ impl Command for Open {
                         }
                     };
 
+                    // Assigning content type should only happen in raw mode. Otherwise, the content
+                    // will potentially be in one of the built-in nushell `from xxx` formats and therefore
+                    // cease to be in the original content-type.... or so I'm told. :)
                     let content_type = if raw {
                         path.extension()
                             .map(|ext| ext.to_string_lossy().to_string())
@@ -283,6 +286,9 @@ fn detect_content_type(extension: &str) -> Option<String> {
     match extension {
         // Per RFC-9512, application/yaml should be used
         "yaml" | "yml" => Some("application/yaml".to_string()),
+        "nu" => Some("application/x-nuscript".to_string()),
+        "json" | "jsonl" | "ndjson" => Some("application/json".to_string()),
+        "nuon" => Some("application/x-nuon".to_string()),
         _ => mime_guess::from_ext(extension)
             .first()
             .map(|mime| mime.to_string()),

crates/nu-command/tests/commands/open.rs

@@ -394,7 +394,7 @@ fn test_content_types_with_open_raw() {
         let result = nu!(cwd: dirs.formats(), "open --raw sample_data.xlsx | metadata");
         assert!(result.out.contains("vnd.openxmlformats-officedocument"));
         let result = nu!(cwd: dirs.formats(), "open --raw sample_def.nu | metadata");
-        assert!(!result.out.contains("content_type"));
+        assert!(result.out.contains("application/x-nuscript"));
         let result = nu!(cwd: dirs.formats(), "open --raw sample.eml | metadata");
         assert!(result.out.contains("message/rfc822"));
         let result = nu!(cwd: dirs.formats(), "open --raw cargo_sample.toml | metadata");