Add example showing first class closure to do (#7473)

# Description

Demonstrates that you can use `do` to execute stored closures and
evaluate their captures properly.

# Tests + Formatting

As an example test increases coverage of the usage to execute first
class closures.
Additional tests using that found in
`tests/shell/pipeline/commands/internal.rs`

crates/nu-command/src/core_commands/do_.rs

@@ -183,6 +183,11 @@ impl Command for Do {
                 example: r#"do { echo hello }"#,
                 result: Some(Value::test_string("hello")),
             },
+            Example {
+                description: "Run a stored first-class closure",
+                example: r#"let text = "I am enclosed"; let hello = {|| echo $text}; do $hello"#,
+                result: Some(Value::test_string("I am enclosed")),
+            },
             Example {
                 description: "Run the closure and ignore both shell and external program errors",
                 example: r#"do -i { thisisnotarealcommand }"#,