Autocomplete design updates

[langermank]

These markup and CSS changes bring the Autocomplete component in closer alignment with PRC and design.

• Uses Overlay and ActionList for the picker styles
• Uses updated FormControl styles to align with Rails forms and PRC.
• Adds support for more features like leading and trailing visuals

In order to safely roll out these changes, this PR moves the current Autocomplete and corresponding Item back to alpha status, also deprecating them. When this release rolls out to GitHub, all implementations of Autocomplete::Beta will be moved back to Autocomplete::Alpha. Feature teams will have the ability to safely update to Beta on their own timeline.

Upgrading from Alpha to Beta

Most of these param changes are auto-fixable when upgrading to beta. Refactoring AutoComplete::Beta::Item is more nuanced.

For Hubbers only: please use this draft PR as a reference for upgrading to Beta.

AutoComplete param changes and additions

old	new	default
is_label_visible	visually_hide_label	false
with_icon	slot <% c.leading_visual_icon(icon: :search) %>
is_clearable	show_clear_button	false
n/a	placeholder
n/a	disabled
n/a	size: :small, :medium, :large	:medium
n/a	inset	false
n/a	monospace	false
n/a	full_width	false

AutoComplete::Item param changes and additions

old	new	default
leading_visual_icon	slot <% c.leading_visual_icon(icon: :search) %>
leading_visual_avatar	slot <% c.leading_visual_avatar(src: "") %>
trailing_visual_icon	slot <% c.trailing_visual_icon(icon: :search) %>
description	slot <% c.description { "Description" } %>
description_variant	:inline, :block	:block

AutoComplete::Item aria-selected view:

CleanShot.2022-07-27.at.17.57.16.mp4

Closes: https://github.com/github/primer/issues/1133