Fix syntax highlighting for not (#12815)

# Description
Fixes #12813 where a panic occurs when syntax highlighting `not`. Also
fixes #12814 where syntax highlighting for `not` no longer works.

# User-Facing Changes
Bug fix.

crates/nu-cli/tests/commands/mod.rs

@@ -0,0 +1 @@
+mod nu_highlight;

crates/nu-cli/tests/commands/nu_highlight.rs

@@ -0,0 +1,7 @@
+use nu_test_support::nu;
+
+#[test]
+fn nu_highlight_not_expr() {
+    let actual = nu!("'not false' | nu-highlight | ansi strip");
+    assert_eq!(actual.out, "not false");
+}

crates/nu-cli/tests/main.rs

@@ -0,0 +1,2 @@
+mod commands;
+mod completions;

crates/nu-parser/src/flatten.rs

@@ -180,12 +180,12 @@ fn flatten_expression_into(
             flatten_expression_into(working_set, op, output);
             flatten_expression_into(working_set, rhs, output);
         }
-        Expr::UnaryNot(expr) => {
+        Expr::UnaryNot(not) => {
             output.push((
                 Span::new(expr.span.start, expr.span.start + 3),
                 FlatShape::Operator,
             ));
-            flatten_expression_into(working_set, expr, output);
+            flatten_expression_into(working_set, not, output);
         }
         Expr::Closure(block_id) => {
             let outer_span = expr.span;