Subissue: Improve support for async command providers

[dlqqq]

Problem

• Handle the case when getChatCommands() and handleChatCommand() are "truly async" and take a long time to resolve. Important for @file: since it needs to talk to the ContentsManager.

Proposed Solution

• Use Promise.all() to start async getChatCommands() tasks in parallel.
• Show chat command suggestions as soon as they resolve, instead of waiting for every provider to finish before opening the chat commands menu.
• Handle loading states in the UI.

Additional context

Initial implementation of chat commands framework: #161.