chat: better problems integration (#241276)

chat: allow referencing and dragging in diagnostics

- There is a new proposal which adds `ChatReferenceDiagnostic` as a
  prompt reference type
- You can now pick "Problems..." as part of the chat attachment context
- You can drag and drop files and individual diagnostics from the
  Problems view into chat. Previously trying to do this would just
  attach the file.

src/vscode-dts/vscode.proposed.chatReferenceDiagnostic.d.ts

@@ -0,0 +1,23 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+declare module 'vscode' {
+
+	export interface ChatPromptReference {
+		/**
+		 * The value of this reference. The `string | Uri | Location` types are used today, but this could expand in the future.
+		 */
+		readonly value: string | Uri | Location | ChatReferenceDiagnostic | unknown;
+	}
+
+	export class ChatReferenceDiagnostic {
+		/**
+		 * All attached diagnostics. An array of uri-diagnostics tuples or an empty array.
+		 */
+		readonly diagnostics: [Uri, Diagnostic[]][];
+
+		protected constructor(diagnostics: [Uri, Diagnostic[]][]);
+	}
+}