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-std/std/iter/mod.nu

@@ -107,16 +107,12 @@ export def scan [ # -> list<any>
     init: any            # initial value to seed the initial state
     fn: closure          # the closure to perform the scan
     --noinit(-n)         # remove the initial value from the result
-] {                      
-    reduce --fold [$init] {|e, acc|
-        let acc_last = $acc | last
-        $acc ++ [($acc_last | do $fn $e $acc_last)]
-    }
-    | if $noinit {
-        $in | skip
-    } else {
-        $in
-    }
+] {
+    generate {|e, acc|
+        let out = $acc | do $fn $e $acc
+        {next: $out, out: $out}
+    } $init
+    | if not $noinit { prepend $init } else { }
 }
 
 # Returns a list of values for which the supplied closure does not