[combobox] Keywords

# Feature request

## Summary

We recently shifted over from `cmdk` to Base UI's `Combobox` — we love it btw! — and one of the features we found ourselves missing was the `keywords` prop: allowing you to specify additional words to match for alongside a specific value

## Examples in other libraries

### `cmdk`

> You can also provide a `keywords` prop to help with filtering. Keywords are trimmed.
>
> ```tsx
> Apple
> ```

## Motivation

For example, in our case, we wanted to match for a user's name and e-mail address

Combobox.Root's `filter` prop allowed us to roll this ourselves, but would be nice just to specify `keywords` as part of `Combobox`'s `item` schema — it was a handy feature of `cmdk`!

cc @moeamaya