Add client-side options support

# Problem
If there are static options passed into the element, the options are deleted on request for filtered options.

# Request
Client-side options should be supported and have filtering available. TBD: Do we allow both client-side (such as default options) *and* server-side options?

# Implementation concerns
## Accessibility
Ensure that any default / client-side options are announced when the input is focused. `screen-reader-announcements.ts` can assist with this.

When the options list isn't visible, an `aria-describedby` should be added to the input pointing to the feedback element. This should only be if the element has client-side options. Remove this `aria-describedby` when the user interacts with any key, and add it back in when the auto-complete-element is blurred. cc @github/accessibility and @jscholes for reference. "aria-live won't cut it" for this scenario

## Developer Notes
There is some initial setup done in `autocomplete.ts` to detect client-side options, but all of this can be removed / reworked, as long as accessibility considerations are met.

Start in autocomplete.ts to review the existing client-side option detection, then read screen-reader-announcements.ts for the accessibility behavior. Trace how options are removed during filtered requests and define the supported interaction between client-side and server-side options. Done means client-side options remain available, filtering works, and the described focus, keyboard, blur, and aria-describedby behavior is covered.

索引モデルが issue の本文から書いたものです。