allow mut to take pipelines (#9658)

# Description

This extends the syntax fix for `let` (#9589) to `mut` as well.

Example: `mut x = "hello world" | str length; print $x`

closes #9634

# User-Facing Changes

`mut` now joins `let` in being able to be assigned from a pipeline

# Tests + Formatting
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crates/nu-command/tests/commands/let_.rs

@@ -50,6 +50,30 @@ fn let_pipeline_allows_in() {
     assert_eq!(actual.out, "21");
 }
 
+#[test]
+fn mut_takes_pipeline() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut x = "hello world" | str length; print $x
+        "#
+    ));
+
+    assert_eq!(actual.out, "11");
+}
+
+#[test]
+fn mut_pipeline_allows_in() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        def foo [] { mut x = $in | str length; print ($x + 10) }; "hello world" | foo
+        "#
+    ));
+
+    assert_eq!(actual.out, "21");
+}
+
 #[ignore]
 #[test]
 fn let_with_external_failed() {