update ast to support output to json (#8962)

# Description
This PR changes the `ast` command to be able to output `--json` as well
as `nuon` (default) with "pretty" and "minified" output. I'm hoping this
functionality will be usable in the vscode extension for semantic
tokenization and highlighting.

# User-Facing Changes
There's a new `--json`/`-j` option. Prior version output of nuon is
maintained as default.

# Tests + Formatting
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crates/nu-protocol/src/ast/block.rs

@@ -1,10 +1,9 @@
+use super::Pipeline;
+use crate::{Signature, Span, VarId};
+use serde::{Deserialize, Serialize};
 use std::ops::{Index, IndexMut};
 
-use crate::{Signature, Span, VarId};
-
-use super::Pipeline;
-
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Block {
     pub signature: Box<Signature>,
     pub pipelines: Vec<Pipeline>,

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

@@ -1,8 +1,8 @@
+use crate::{ast::Expression, engine::StateWorkingSet, Span, VarId};
+use serde::{Deserialize, Serialize};
 use std::ops::{Index, IndexMut};
 
-use crate::{ast::Expression, engine::StateWorkingSet, Span, VarId};
-
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum Redirection {
     Stdout,
     Stderr,
@@ -10,7 +10,7 @@ pub enum Redirection {
 }
 
 // Note: Span in the below is for the span of the connector not the whole element
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum PipelineElement {
     Expression(Option<Span>, Expression),
     Redirection(Span, Redirection, Expression),
@@ -106,7 +106,7 @@ impl PipelineElement {
     }
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Pipeline {
     pub elements: Vec<PipelineElement>,
 }