lib: Support headers with TypeaheadSelect

[mvollmer]

• lib: Introduce our own TypeaheadSelect and use it everywhere #21175
• tests for the new TypeaheadSelect in general
• Style the headers nicely

I have styled the headers like PF menu groups:

[mvollmer]

@garrett, I need your design input for how the headers should look.

PF5 headers look like this:

[cockpituous]

This added line is not executed by any test.

[garrett]

@garrett, I need your design input for how the headers should look.

PF5 headers look like this:

Yes, that's how PF5 headers look.

1. What are you looking for specifically? I'm not sure what you're asking.
2. How would I check this out to verify that it looks like PF5? (Like, which page, modal, widget?) I've poked around after building the branch and it's not clear what I should be looking at.

[mvollmer]

@garrett, I need your design input for how the headers should look.
PF5 headers look like this:

Yes, that's how PF5 headers look.

1. What are you looking for specifically? I'm not sure what you're asking.

This PR implements its own headers, and doesn't use the PF group headers. (For technical reasons, the typeahead template is much easier to adapt to special entries in a flat list (the headers) than to dealing with a recursive structure (PFs select groups)).

So the question is, how should the headers look? We can make them look identical to PF5, or easily improve upon the looks, if we want.

2. How would I check this out to verify that it looks like PF5? (Like, which page, modal, widget?) I've poked around after building the branch and it's not clear what I should be looking at.

It's in the "Development Playground", sub-page "React Patterns": https://host:9090/playground/react-patterns