Add a Twig component to display flash messages

[javiereguiluz]

We're going to transform the entire UI using Twig Components. We've already have 2 components and this adds the third one.

It's called Banner and it can render flash messages / notifications.

Some comments:

• I called it Banner to follow GitHub's Primer component naming (https://primer.style/components/banner) I like GitHub components and I'm getting many ideas from them.
• The prop names withDismissButton and variant are also inspired/copied from GtiHub components
• I created an Enum (the first one in this bundle!; this is fine because we now require PHP 8.1 and enums are supported there) to define the possible values of variant. I opted for the namespace Config\Enum\... because we already have Config\Option\... for the constant-based classes. The idea is to turn (in the future) the constants into enums, so we need a different namespace. Do you like Enum\ or do you prefer a different name?

Thanks!

[dragosprotung]

* I created an `Enum` (the first one in this bundle!; this is fine because we now require PHP 8.1 and enums re supported there) to define the possible values of `variant`. I opted for the namespace `Config\Enum\...` because we already have `Config\Option\...` for the constant-based classes. The idea is to turn (in the future) the constants into enums, so we need a different namespace. Do you like `Enum\` or do you prefer a different name?

Why not keep BannerVariant in the same namespace at the twig component ?

[javiereguiluz]

I don't want to use the exact same namespace because "component" != "component option".

But your comment gave me an idea: use the Config/Option namespace inside Twig/Component namespace

[dragosprotung]

I don't want to use the exact same namespace because "component" != "component option".

But your comment gave me an idea: use the Config/Option namespace inside Twig/Component namespace

Yes, "component" != "component option", but will this option ever be used outside of configuring the component, or another component ?

[javiereguiluz]

Folks, thanks for the comments and reviews. I made most of the changes you proposed and also changed other things thanks to the comments shared on Symfony Slack. Thanks 🙌