Improve Dict\merge

[veewee]

Describe the bug
Currently Dict\merge is using the spread operator internally.
This behaves weird on numeric or numeric-string keys:

https://3v4l.org/gV0V8

var_dump([
    ...['1000' => 'hello', 'non-numeric' => 'world', 0 => 'you too']
]);

array(3) {
  [0]=>
  string(5) "hello"
  ["non-numeric"]=>
  string(5) "world"
  [1]=>
  string(7) "you too"
}

Expected behavior

Since the merge is being done on dict's, it should merge based on exact key and not try to merge as a list.
We already have Vec\concat for concatenating lists.

array(3) {
  ["1000"]=>
  string(5) "hello"
  ["non-numeric"]=>
  string(5) "world"
  [0]=>
  string(7) "you too"
}

[veewee]

Oh sorry, opened up an issue to fast!
It seems to be working just fine - I'll create a PR with an additional testcase to prove.