Chat - Input area usability improvements

I originally posted this on Reddit, but I guess it's worth logging it as a GitHub issue for better tracking.

Feature-wise Copilot in VS Code is already pretty solid. We have everything we need. What I think is lacking, compared to the rest of the workbench, is the chat input UX. Especially when working with references/mentions, it does get frustrating. Here is a brief list of issues/improvements that come to mind every time I use the input area:

- The `#` reference should not be completely discarded when a character that's preceding it gets changed.

- The `#` reference should always be stable until it gets removed with a backspace. An incorrect file reference should not invalidate it, but should simply show a red marker and disable the send button.

- The `#` reference should be editable in isolation: when clicking over it, it should open the palette as with the explicit context, to allow the user to change the reference. Right now a CTRL+Click or Context menu do nothing.

- `#` and `@` should not be interchangeable. Right now they mostly show the same items, and it's extremely confusing.

Feel free to split this up into multiple sub-issues if required. Or, I can do it if needed.