++= 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/mod.rs

@@ -0,0 +1 @@
+mod append_assign;