explain command (#7957)

# Description

The purpose of this PR is to introduce the `inspect` command. A command
that is used to inspect, but not run, a pipeline to ensure everything
looks right. This is meant as a debugging tool. This is some hackery, so
don't laugh. :)

![image](https://user-images.githubusercontent.com/343840/217896776-99c6bece-172c-4d3d-8cec-dda85d37cada.png)

crates/nu-protocol/src/ast/pipeline.rs

@@ -23,6 +23,19 @@ pub enum PipelineElement {
 }
 
 impl PipelineElement {
+    pub fn expression(&self) -> &Expression {
+        match self {
+            PipelineElement::Expression(_, expression) => expression,
+            PipelineElement::Redirection(_, _, expression) => expression,
+            PipelineElement::SeparateRedirection {
+                out: (_, expression),
+                ..
+            } => expression,
+            PipelineElement::And(_, expression) => expression,
+            PipelineElement::Or(_, expression) => expression,
+        }
+    }
+
     pub fn span(&self) -> Span {
         match self {
             PipelineElement::Expression(None, expression) => expression.span,