This PR adds support for keyboard navigation to the Autocomplete widget:

Specifically it adds new intents AutocompletePreviousOptionIntent and AutocompleteNextOptionIntent bound to the up arrow and down arrow keys respectively. These are handled by actions that move a highlight up and down the options list. If the field is submitted (usually with the enter key) it will replace the text with the highlighted option.

If the app provides a custom optionsViewBuilder to the Autocomplete widget, it will need to manage the display of the highlighted option manually. To do this they can get the index of the highlighted option with the inherited widget AutocompleteHighlightedOption. This can be accessed in the builder with:

final highlightedIndex = AutocompleteHighlightedOption.of(context);

Fixes: #82783

I have added several tests to verify the keyboard navigation is working correctly.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.