[10.x] Add @slots directive for optional component slots

[indykoning]

This PR will add a @slots blade directive mainly for use in anonymous components.
It adds a mechanism to declare optional slots at the top of your components which will automatically be transformed into ComponentSlots when they are a string or do not exist.

The issue this solves is, say you're building a dialog component with text and a button which you can optionally change the styling and contents of. You might have your template look like this.

<dialog {{ $attributes->class('absolute inset') }}>
	{{ $slot }}
	<div>
		<button {{ $button->attributes->class('bg-green-200 ml-0 mr-auto') }}>{{ $button ?? 'close' }}</button>
	</div>
</dialog>

However this will never work as button is null and $button->attributes will throw an error let alone merging classes.
The options you have are adding php at the top or coalescing the $button->attributes->class and copy-pasting your classes.

The solution this offers is the following template

@slots(['button' => ['contents' => 'close']])
<dialog {{ $attributes->class('absolute inset') }}>
	{{ $slot }}
	<div>
		<button {{ $button->attributes->class('bg-green-200 ml-0 mr-auto') }}>{{ $button }}</button>
	</div>
</dialog>

Which will define all optional slots used in the code clearly at the top, and allow us to set default data to be overwritten if the named slot is used.

This is not breaking as it is a new feature which affects nothing if it is not used.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.

[royduin]

If somebody wants it: https://github.com/rapidez/blade-directives