add ability to set metadata (#12564)

# Description

This PR adds the ability to set metadata. This is especially useful for
activating LS_COLORS when using table literals.


![image](https://github.com/nushell/nushell/assets/343840/feef6433-f592-43ea-890a-38cb2df35686)

You can also set the filepath metadata, although I'm not really user how
useful this is. We may end up removing this option entirely.
```nushell
❯ "crates" | metadata set --datasource-filepath $'(pwd)/crates' | metadata
╭────────┬───────────────────────────────────╮
│ source │ /Users/fdncred/src/nushell/crates │
╰────────┴───────────────────────────────────╯
```

No file paths are checked. You could also do this.
```nushell
❯ "crates" | metadata set --datasource-filepath $'a/b/c/d/crates' | metadata
╭────────┬────────────────╮
│ source │ a/b/c/d/crates │
╰────────┴────────────────╯
```

The command name and parameter names are still WIP. We could change
them.

There are currently 3 kinds of metadata in nushell.
```rust
pub enum DataSource {
    Ls,
    HtmlThemes,
    FilePath(PathBuf),
}
```

I've skipped adding `HtmlThemes` because it seems to be specific to our
`to html` command only.

crates/nu-command/src/debug/metadata_set.rs

@@ -0,0 +1,92 @@
+use nu_engine::command_prelude::*;
+use nu_protocol::{DataSource, PipelineMetadata};
+
+#[derive(Clone)]
+pub struct MetadataSet;
+
+impl Command for MetadataSet {
+    fn name(&self) -> &str {
+        "metadata set"
+    }
+
+    fn usage(&self) -> &str {
+        "Set the metadata for items in the stream."
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("metadata set")
+            .input_output_types(vec![(Type::Any, Type::Any)])
+            .switch(
+                "datasource-ls",
+                "Assign the DataSource::Ls metadata to the input",
+                Some('l'),
+            )
+            .named(
+                "datasource-filepath",
+                SyntaxShape::Filepath,
+                "Assign the DataSource::FilePath metadata to the input",
+                Some('f'),
+            )
+            .allow_variants_without_examples(true)
+            .category(Category::Debug)
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let head = call.head;
+        let ds_fp: Option<String> = call.get_flag(engine_state, stack, "datasource-filepath")?;
+        let ds_ls = call.has_flag(engine_state, stack, "datasource-ls")?;
+
+        match (ds_fp, ds_ls) {
+            (Some(path), false) => {
+                let metadata = PipelineMetadata {
+                    data_source: DataSource::FilePath(path.into()),
+                };
+                Ok(input.into_pipeline_data_with_metadata(metadata, engine_state.ctrlc.clone()))
+            }
+            (None, true) => {
+                let metadata = PipelineMetadata {
+                    data_source: DataSource::Ls,
+                };
+                Ok(input.into_pipeline_data_with_metadata(metadata, engine_state.ctrlc.clone()))
+            }
+            _ => Err(ShellError::IncorrectValue {
+                msg: "Expected either --datasource-ls(-l) or --datasource-filepath(-f)".to_string(),
+                val_span: head,
+                call_span: head,
+            }),
+        }
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Set the metadata of a table literal",
+                example: "[[name color]; [Cargo.lock '#ff0000'] [Cargo.toml '#00ff00'] [README.md '#0000ff']] | metadata set --datasource-ls",
+                result: None,
+            },
+            Example {
+                description: "Set the metadata of a file path",
+                example: "'crates' | metadata set --datasource-filepath $'(pwd)/crates' | metadata",
+                result: None,
+            },
+        ]
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(MetadataSet {})
+    }
+}

crates/nu-command/src/debug/mod.rs

@@ -5,6 +5,7 @@ mod info;
 mod inspect;
 mod inspect_table;
 mod metadata;
+mod metadata_set;
 mod profile;
 mod timeit;
 mod view;
@@ -19,6 +20,7 @@ pub use info::DebugInfo;
 pub use inspect::Inspect;
 pub use inspect_table::build_table;
 pub use metadata::Metadata;
+pub use metadata_set::MetadataSet;
 pub use profile::DebugProfile;
 pub use timeit::TimeIt;
 pub use view::View;

crates/nu-command/src/default_context.rs

@@ -143,6 +143,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             Explain,
             Inspect,
             Metadata,
+            MetadataSet,
             TimeIt,
             View,
             ViewFiles,