Use 'insert' mode for # in chat (microsoft#255580)

I originally changed this to fix some issue with `#file:` which doesn't exist anymore, and I don't think there is another scenario like that, and this behavior seems more expected.
Fix microsoft/vscode-copilot#14764

Author: roblourens

src/vs/workbench/contrib/chat/browser/chatInputPart.ts

@@ -1080,7 +1080,7 @@ export class ChatInputPart extends Disposable implements IHistoryNavigationWidge
 			showSnippets: false,
 			showWords: true,
 			showStatusBar: false,
-			insertMode: 'replace',
+			insertMode: 'insert',
 		};
 		options.scrollbar = { ...(options.scrollbar ?? {}), vertical: 'hidden' };
 		options.stickyScroll = { enabled: false };