++= appendAssign operator (#7346) (#7354)

# Description

Closes  https://github.com/nushell/nushell/issues/7346



# Tests + Formatting
```
> mut a = [1 2 3]
> $a ++= [4 5 6]
> $a
[1 2 3 4 5 6]
```

crates/nu-command/tests/commands/assignment/append_assign.rs

@@ -0,0 +1,108 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn append_assign_int() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [1 2];
+            $a ++= [3 4];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            [1 2 3 4]
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_string() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [a b];
+            $a ++= [c d];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            [a b c d]
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_any() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [1 2 a];
+            $a ++= [b 3];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            [1 2 a b 3]
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_both_empty() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [];
+            $a ++= [];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            []
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_type_mismatch() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [1 2];
+            $a ++= [a];
+        "#
+    ));
+
+    assert!(actual
+        .err
+        .contains("expected list<int>, found list<string>"));
+}