Make plugins able to find and call other commands (#13407)

# Description

Adds functionality to the plugin interface to support calling internal
commands from plugins. For example, using `view ir --json`:

```rust
let closure: Value = call.req(0)?;

let Some(decl_id) = engine.find_decl("view ir")? else {
    return Err(LabeledError::new("`view ir` not found"));
};

let ir_json = engine.call_decl(
    decl_id,
    EvaluatedCall::new(call.head)
        .with_named("json".into_spanned(call.head), Value::bool(true, call.head))
        .with_positional(closure),
    PipelineData::Empty,
    true,
    false,
)?.into_value()?.into_string()?;

let ir = serde_json::from_value(&ir_json);

// ...
```

# User-Facing Changes

Plugin developers can now use `EngineInterface::find_decl()` and
`call_decl()` to call internal commands, which could be handy for
formatters like `to csv` or `to nuon`, or for reflection commands that
help gain insight into the engine.

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
- [ ] release notes
- [ ] update plugin protocol documentation: `FindDecl`, `CallDecl`
engine calls; `Identifier` engine call response

crates/nu_plugin_example/src/commands/call_decl.rs

@@ -0,0 +1,78 @@
+use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
+use nu_protocol::{
+    IntoSpanned, LabeledError, PipelineData, Record, Signature, Spanned, SyntaxShape, Value,
+};
+
+use crate::ExamplePlugin;
+
+pub struct CallDecl;
+
+impl PluginCommand for CallDecl {
+    type Plugin = ExamplePlugin;
+
+    fn name(&self) -> &str {
+        "example call-decl"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .required(
+                "name",
+                SyntaxShape::String,
+                "the name of the command to call",
+            )
+            .optional(
+                "named_args",
+                SyntaxShape::Record(vec![]),
+                "named arguments to pass to the command",
+            )
+            .rest(
+                "positional_args",
+                SyntaxShape::Any,
+                "positional arguments to pass to the command",
+            )
+    }
+
+    fn usage(&self) -> &str {
+        "Demonstrates calling other commands from plugins using `call_decl()`."
+    }
+
+    fn extra_usage(&self) -> &str {
+        "
+The arguments will not be typechecked at parse time. This command is for
+demonstration only, and should not be used for anything real.
+"
+        .trim()
+    }
+
+    fn run(
+        &self,
+        _plugin: &ExamplePlugin,
+        engine: &EngineInterface,
+        call: &EvaluatedCall,
+        input: PipelineData,
+    ) -> Result<PipelineData, LabeledError> {
+        let name: Spanned<String> = call.req(0)?;
+        let named_args: Option<Record> = call.opt(1)?;
+        let positional_args: Vec<Value> = call.rest(2)?;
+
+        let decl_id = engine.find_decl(&name.item)?.ok_or_else(|| {
+            LabeledError::new(format!("Can't find `{}`", name.item))
+                .with_label("not in scope", name.span)
+        })?;
+
+        let mut new_call = EvaluatedCall::new(call.head);
+
+        for (key, val) in named_args.into_iter().flatten() {
+            new_call.add_named(key.into_spanned(val.span()), val);
+        }
+
+        for val in positional_args {
+            new_call.add_positional(val);
+        }
+
+        let result = engine.call_decl(decl_id, new_call, input, true, false)?;
+
+        Ok(result)
+    }
+}

crates/nu_plugin_example/src/commands/mod.rs

@@ -13,11 +13,13 @@ pub use three::Three;
 pub use two::Two;
 
 // Engine interface demos
+mod call_decl;
 mod config;
 mod disable_gc;
 mod env;
 mod view_span;
 
+pub use call_decl::CallDecl;
 pub use config::Config;
 pub use disable_gc::DisableGc;
 pub use env::Env;

crates/nu_plugin_example/src/lib.rs

@@ -27,6 +27,7 @@ impl Plugin for ExamplePlugin {
             Box::new(Env),
             Box::new(ViewSpan),
             Box::new(DisableGc),
+            Box::new(CallDecl),
             // Stream demos
             Box::new(CollectBytes),
             Box::new(Echo),