Allow attaching libraries, js, css with hook_theme.

[laryn]

Description of the need

During the dev call today, we discussed potential benefits of allowing the attachment of libraries, js, css with hook_theme, rather than having to manually add them with backdrop_add_library() within a preprocess function, for example.

Proposed solution

Allowed #attached values to be used with hook_theme implementations, so that CSS and JS can be specified and be automatically added (only when the theme implementation is actually used). This will allow an easy way to specify libraries, JS and CSS and have it optimized to be used only when needed. Alters will also allow other implementations to make adjustments if needed.

[indigoxela]

Allowed #attached values to be used with hook_theme...

Hm, #attached is commonly used with forms or elements (renderable arrays), but in info-hooks? 🤔 @laryn do you have an example, where #attached also works with existing info hooks?

Or, do you mind to elaborate, why you can't attach libs within the actual theme implementation?

[laryn]

I think this would probably be a new way of using it. We had some discussion of it here that fleshed it out a bit, a few weeks ago:

• https://www.youtube.com/live/3G5N65Ewcko?si=M0gb5q51LzuB8w9P&t=1792

I am hoping to be able to make a PR for this but if people have opinions in the meantime, this is the place to collect and discuss them.