feat: replace unfold with from_fn for the generate command (#13299)

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This PR should close #13247 

# Description
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- The deprecated `itertools::unfold` function is replaced with
`std::iter::from_fn` for the generate command.
- The mutable iterator state is no longer passed as an argument to
`from_fn` but it gets captured with the closure's `move`.

# User-Facing Changes
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No user facing changes

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Tests for the generate command are passing locally.

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---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

crates/nu-command/src/generators/generate.rs

@@ -1,4 +1,3 @@
-use itertools::unfold;
 use nu_engine::{command_prelude::*, ClosureEval};
 use nu_protocol::engine::Closure;
 
@@ -81,7 +80,8 @@ used as the next argument to the closure, otherwise generation stops.
         // A type of Option<S> is used to represent state. Invocation
         // will stop on None. Using Option<S> allows functions to output
         // one final value before stopping.
-        let iter = unfold(Some(initial), move |state| {
+        let mut state = Some(initial);
+        let iter = std::iter::from_fn(move || {
             let arg = state.take()?;
 
             let (output, next_input) = match closure.run_with_value(arg) {
@@ -160,7 +160,7 @@ used as the next argument to the closure, otherwise generation stops.
             // We use `state` to control when to stop, not `output`. By wrapping
             // it in a `Some`, we allow the generator to output `None` as a valid output
             // value.
-            *state = next_input;
+            state = next_input;
             Some(output)
         });