++= 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/src/core_commands/help_operators.rs

@@ -81,6 +81,13 @@ fn generate_operator_info() -> Vec<OperatorInfo> {
             description: "Adds a value to a variable.".into(),
             precedence: 10,
         },
+        OperatorInfo {
+            op_type: "Assignment".into(),
+            operator: "++=".into(),
+            name: "AppendAssign".into(),
+            description: "Appends a list or a value to a variable.".into(),
+            precedence: 10,
+        },
         OperatorInfo {
             op_type: "Assignment".into(),
             operator: "-=".into(),