feat(where): Support stored closure (#15697)

# Description

- `where` used to be able to filter using stored closures at some point
using a flag.
  > #5955
- This was later removed and `filter` command added to cover the use
case.
  > #7365

This PR once again allows using `where` with closures stored in
variables.

```nushell
let cond = { $in mod 2 == 0 }
1..10 | where $cond
# => ╭───┬────╮
# => │ 0 │  2 │
# => │ 1 │  4 │
# => │ 2 │  6 │
# => │ 3 │  8 │
# => │ 4 │ 10 │
# => ╰───┴────╯

let nested = {cond: { $in mod 2 == 0 }}
1..10 | where $nested.cond 
# => ╭───┬────╮
# => │ 0 │  2 │
# => │ 1 │  4 │
# => │ 2 │  6 │
# => │ 3 │  8 │
# => │ 4 │ 10 │
# => ╰───┴────╯
``` 

This does not interfere with using `$it` or one of its fields as the
condition.
```nushell
[[name state]; [foo true] [bar false] [baz true] [quox false]]
| where $it.state
# => ╭───┬──────┬───────╮
# => │ # │ name │ state │
# => ├───┼──────┼───────┤
# => │ 0 │ foo  │ true  │
# => │ 1 │ baz  │ true  │
# => ╰───┴──────┴───────╯
``` 

This change obsoletes `filter`, deprecate it in the future?

# User-Facing Changes

# Tests + Formatting

Added examples and tests.

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


# After Submitting

---------

Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>

crates/nu-command/src/filters/where_.rs

@@ -35,7 +35,10 @@ not supported."#
             ])
             .required(
                 "row_condition",
-                SyntaxShape::RowCondition,
+                SyntaxShape::OneOf(vec![
+                    SyntaxShape::RowCondition,
+                    SyntaxShape::Closure(Some(vec![SyntaxShape::Any])),
+                ]),
                 "Filter condition.",
             )
             .allow_variants_without_examples(true)
@@ -122,6 +125,21 @@ not supported."#
                 example: "ls | where name =~ '(?i)readme'",
                 result: None,
             },
+            Example {
+                description: "Filter rows of a table according to a stored condition",
+                example: "let cond = {|x| $x.a > 1}; [{a: 1} {a: 2}] | where $cond",
+                result: Some(Value::test_list(vec![Value::test_record(record! {
+                    "a" => Value::test_int(2),
+                })])),
+            },
+            Example {
+                description: "List all numbers above 3, using an existing closure condition",
+                example: "let a = {$in > 3}; [1, 2, 5, 6] | where $a",
+                result: Some(Value::test_list(vec![
+                    Value::test_int(5),
+                    Value::test_int(6),
+                ])),
+            },
         ]
     }
 }

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

@@ -28,6 +28,12 @@ fn where_inside_block_works() {
     assert_eq!(actual.out, "closure");
 }
 
+#[test]
+fn it_inside_complex_subexpression() {
+    let actual = nu!(r#"1..10 | where [($it * $it)].0 > 40  | to nuon"#);
+    assert_eq!(actual.out, r#"[7, 8, 9, 10]"#)
+}
+
 #[test]
 fn filters_with_0_arity_block() {
     let actual = nu!("[1 2 3 4] | where {|| $in < 3 } | to nuon");
@@ -209,3 +215,16 @@ fn has_operator() {
     let actual = nu!(r#"{foo: 1} has bar "#);
     assert_eq!(actual.out, "false");
 }
+
+#[test]
+fn stored_condition() {
+    let actual = nu!(r#"let cond = { $in mod 2 == 0 }; 1..10 | where $cond | to nuon"#);
+    assert_eq!(actual.out, r#"[2, 4, 6, 8, 10]"#)
+}
+
+#[test]
+fn nested_stored_condition() {
+    let actual =
+        nu!(r#"let nested = {cond: { $in mod 2 == 0 }}; 1..10 | where $nested.cond | to nuon"#);
+    assert_eq!(actual.out, r#"[2, 4, 6, 8, 10]"#)
+}