Add input support to generate (#14804)

- closes #8523 

# Description

This PR adds pipeline input support to `generate`.
- Without input, `generate` keeps its current behavior.
- With input, each invocation of the closure is provided an item from
the input stream as pipeline input (`$in`). If/when the input stream
runs out, `generate` also stops.

Before this PR, there is no filter command that is both stateful _and_
streaming.

This PR also refactors `std/iter scan` to use `generate`, making it
streaming and more performant over larger inputs.

# User-Facing Changes
- `generate` now supports pipeline input, passing each element to the
closure as `$in` until it runs out
- `std/iter scan` is now streaming

# Tests + Formatting
Added tests to validate the new feature.

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
N/A

crates/nu-command/tests/commands/generate.rs

@@ -148,3 +148,25 @@ fn generate_raise_error_on_no_default_parameter_closure_and_init_val() {
     ));
     assert!(actual.err.contains("The initial value is missing"));
 }
+
+#[test]
+fn generate_allows_pipeline_input() {
+    let actual = nu!(r#"[1 2 3] | generate {|e, x=null| {out: $e, next: null}} | to nuon"#);
+    assert_eq!(actual.out, "[1, 2, 3]");
+}
+
+#[test]
+fn generate_with_input_is_streaming() {
+    let actual = nu!(pipeline(
+        r#"
+        1..10
+        | each {|x| print -en $x; $x}
+        | generate {|e, sum=0| let sum = $e + $sum; {out: $sum, next: $sum}}
+        | first 5
+        | to nuon
+        "#
+    ));
+
+    assert_eq!(actual.out, "[1, 3, 6, 10, 15]");
+    assert_eq!(actual.err, "12345");
+}