Implement mapByKey

[driesvints]

This PR adds a new array helper and collection method called mapByKey. With this method instead of this:

$result = Arr::map($array, function ($value, $key) {
    return in_array($key, ['foo', 'bar']) ? strtolower($value) : $value;
});

You can do:

$result = Arr::mapByKey($array, ['foo', 'bar'], fn ($value) => strtolower($value));

So basically target any key in a given item on an array and manipulate it.

Things not added yet which I'd love to get some help with:

• Verify if LazyCollection support is implemented correctly
• Generics in docblocks
• Deep nested key targeting with dot notation
• Enumerable contract method (can be added on master once it's merged upstream)

[driesvints]

I'm not 100% sure this is the correct way to go about adding a method to LazyCollection.

[driesvints]

@josepostiga would be cool if you could give some feedback on what can be improved here.

[shaedrich]

This PR adds a new array helper and collection method called mapByKey

I'm not sure if that is the ideal naming. To me, it does not exactly convey what it does. Actually, it's doing something more along the lines of mapSpecificKeys

[josepostiga]

@josepostiga would be cool if you could give some feedback on what can be improved here.

Sorry, didn't mean to cause entropy in your development/thinking process 😅

I think the behavior is interesting. I just think the name could be a little different? Maybe mapFilteringByKey?

[RVxLab]

I think this could be a very nice feature. I’m just a bit confused by the name. To me, on its face, looks like another way to map but the key is the first argument.

What about mapOnly for the name? It would fit the names on Eloquent models, collections and the array helper. Inversely that could lead to mapExcept.

[taylorotwell]

Going to skip this one for now.