Add field group preprocess

I'm not sure how to implement this.

I started with a HOOK_field_group_pre_render_alter(). $element['component'] is available to populate, and I can get it to load the preprocess file for the groupname. But I think it should also look for files for each field, so it can work like the field preprocessor, but for the groups props/slots.

This is a good idea since field_group is a popular module and is used often with SDC.

I'll continue off what you started and perform some tests.

Took a look, tested it, and I think what was done was already sufficient.

When choosing to render a field group as a component, I don't think we need to look for files for each field. This is because the group as a whole will be rendered as the component, and all needed preprocessing should be able to be achieved within the one file. In the loaded file we'll have access to all props and slots for the given group.

Did you think of a use case where individual files would be needed? I might be missing something.

I'm a little fuzzy on this from when I last looked at it, it's possible I overlooked how it's supposed to work. I'll have to look into this closer next time I encounter the issue.

That's fine! I've gone ahead and opened a merge request with the work done so far. I think it's a good first version to at least support field group integration.

Good to go in release 1.0.0-alpha2.

We can open a new issue if what was discussed in #2 & #4 becomes a need down the line.