[EuiSuggest] Rebuild using EuiSelectable

[myasonik]

The existing EuiSuggest has quite a few a11y shortcomings (see #2404 for the original remediation plan).

Instead of effectively rewriting it from scratch, it should use EuiSelectable to inherit all the a11y work that went into it and to consolidate implementations of similar components.

On top of EuiSelectable, EuiSuggest largely provides:

1. An opinionated rendering of individual items
2. Some more states of the search field

I think the EUI team prefers to keep the EuiSuggest component and to wrap EuiSelectable within it but another implementation option is to add a "renderAs" prop (or something along those lines) to EuiSelectable to adjust styling.

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[thompsongl]

I've started exploring the conversion to EuiSelectable and the path looks straightforward, but I have questions about the purpose of EuiSuggest. Mainly, it is currently a fully controlled component (i.e., it does not handle filtering the list or managing selected items, but relies on a parent controller to do so). Is this a paradigm we want to continue? Are we converting to EuiSelectable strictly for the a11y benefits? Or is part of using EuiSelectable is to enable its search, filter, and selection features?

cc// @cchaos

[cchaos]

This one is tricky and I'm honestly not sure if this component is even used anywhere (yet). But my guess is that we'd definitely have to support a mechanism of custom search queries and not just plain text filtering. what we'd probably do is just use the selectable list for rendering the items and keep the search input separate.

[thompsongl]

what we'd probably do is just use the selectable list for rendering the items and keep the search input separate.

That's my read as well. I'll probably put up a PR with this type of conversion and we can discuss more there. At the very least it's a step in the right direction.