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-engine/src/interface/mod.rs

@@ -1316,6 +1316,22 @@ pub(crate) fn handle_engine_call(
         } => context
             .eval_closure(closure, positional, input, redirect_stdout, redirect_stderr)
             .map(EngineCallResponse::PipelineData),
+        EngineCall::FindDecl(name) => context.find_decl(&name).map(|decl_id| {
+            if let Some(decl_id) = decl_id {
+                EngineCallResponse::Identifier(decl_id)
+            } else {
+                EngineCallResponse::empty()
+            }
+        }),
+        EngineCall::CallDecl {
+            decl_id,
+            call,
+            input,
+            redirect_stdout,
+            redirect_stderr,
+        } => context
+            .call_decl(decl_id, call, input, redirect_stdout, redirect_stderr)
+            .map(EngineCallResponse::PipelineData),
     }
 }