[7.x] Add undot array helper

[divspace]

Adds the inverse to the dot array helper by expanding array keys with "dot" notation into multi-dimensional arrays.

Besides adding the inverse functionality, there are times when certain functions/methods in your code may not allow multi-dimensional arrays and you need a way to pass flattened "dot" notated arrays to them (or maybe you find the syntax easier to write). For example, if you were to add a message with something like:

$messages = new MessageBag([
    'error.code' => 403,
    'error.message' => 'Unauthorized request',
]);

You could store those messages correctly, and after retrieving them, simply undot the messages to get a proper array:

[
  'error' => [
    'code' => 403,
    'message' => 'Unauthorized request'
  ]
]

This does not break existing functionality since it's a new helper and tests have been provided.

This is my first pull request ever to a project that isn't work related, so hopefully I did this right (even watched Jeffrey Way's video on Laracasts). I think I spent more time writing the method's description than anything else 😬

[mnabialek]

It was already proposed and rejected a few times:

• [PROPOSAL] Helpers : array_elevate #4292
• Added array_undot function #4735
• [5.2] Add Arr::unDot() method #13897

[divspace]

Oh, didn’t even think to search first. Oh well, close this then I’ll just keep extending Arr out 🤟