Match ++= capabilities with ++ (#11130)

Allow `++=` to work in all situations `++` does, namely for appending
single elements: `$list ++= 1`.

Resolve #11087

# Description

Bring `++=` to parity with `++`.

# User-Facing Changes

It is now possible to do `$list ++= 1` (appending a single element).
Similarly, this can be done:

```Nushell
~> mut a = [1]
~> $a ++= 2
~> a
╭───┬───╮
│ 0 │ 1 │
│ 1 │ 2 │
╰───┴───╯
```

# Tests + Formatting

Added two tests:

- `commands::assignment::append_assign::append_assign_single_element`
- `commands::assignment::append_assign::append_assign_to_single_element`

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

@@ -5,16 +5,10 @@ fn append_assign_int() {
     let actual = nu!(r#"
             mut a = [1 2];
             $a ++= [3 4];
-            $a
+            $a == [1 2 3 4]
         "#);
 
-    let expected = nu!(r#"
-            [1 2 3 4]
-        "#);
-
-    print!("{}", actual.out);
-    print!("{}", expected.out);
-    assert_eq!(actual.out, expected.out);
+    assert_eq!(actual.out, "true")
 }
 
 #[test]
@@ -22,16 +16,10 @@ fn append_assign_string() {
     let actual = nu!(r#"
             mut a = [a b];
             $a ++= [c d];
-            $a
+            $a == [a b c d]
         "#);
 
-    let expected = nu!(r#"
-            [a b c d]
-        "#);
-
-    print!("{}", actual.out);
-    print!("{}", expected.out);
-    assert_eq!(actual.out, expected.out);
+    assert_eq!(actual.out, "true")
 }
 
 #[test]
@@ -39,16 +27,10 @@ fn append_assign_any() {
     let actual = nu!(r#"
             mut a = [1 2 a];
             $a ++= [b 3];
-            $a
+            $a == [1 2 a b 3]
         "#);
 
-    let expected = nu!(r#"
-            [1 2 a b 3]
-        "#);
-
-    print!("{}", actual.out);
-    print!("{}", expected.out);
-    assert_eq!(actual.out, expected.out);
+    assert_eq!(actual.out, "true")
 }
 
 #[test]
@@ -56,16 +38,10 @@ fn append_assign_both_empty() {
     let actual = nu!(r#"
             mut a = [];
             $a ++= [];
-            $a
+            $a == []
         "#);
 
-    let expected = nu!(r#"
-            []
-        "#);
-
-    print!("{}", actual.out);
-    print!("{}", expected.out);
-    assert_eq!(actual.out, expected.out);
+    assert_eq!(actual.out, "true")
 }
 
 #[test]
@@ -73,8 +49,40 @@ fn append_assign_type_mismatch() {
     let actual = nu!(r#"
             mut a = [1 2];
             $a ++= [a];
-            $a | to json -r;
+            $a == [1 2 "a"]
         "#);
 
-    assert_eq!(actual.out, r#"[1,2,"a"]"#);
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn append_assign_single_element() {
+    let actual = nu!(r#"
+            mut a = ["list" "and"];
+            $a ++= "a single element";
+	    $a == ["list" "and" "a single element"]
+        "#);
+
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn append_assign_to_single_element() {
+    let actual = nu!(r#"
+            mut a = "string";
+            $a ++= ["and" "the" "list"];
+	    $a == ["string" "and" "the" "list"]
+        "#);
+
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn append_assign_single_to_single() {
+    let actual = nu!(r#"
+            mut a = 1;
+            $a ++= "and a single element";
+        "#);
+
+    assert!(actual.err.contains("nu::parser::unsupported_operation"));
 }