Make inline suggestion widget show item for in-progress AI autocomplete

[owtaylor]

If the suggestion widget pops up while our autocompletion model is still working, then the AI results are completely hidden and not seen unless the user types more characters that match the AI completion, or the user arrows down to the matching intellisense completion.

@halfline is trying an approach where we contribute to the suggestion widget an entry that:

• shows progress while the AI is thinking
• shows the AI's results afterwards