Allow pressing escape to cancel the current assistant generation (#10987)

If the assistant has already emitted some text, we will leave the
assistant message but maintain the cursor on the previous user message,
so that the user can easily discard the message by submitting again.

If no output was emitted yet, we simply delete the empty assistant
message.

Release Notes:

- N/A

assets/keymaps/default-macos.json

@@ -212,7 +212,8 @@
     "context": "AssistantChat > Editor", // Used in the assistant2 crate
     "bindings": {
       "enter": ["assistant2::Submit", "Simple"],
-      "cmd-enter": ["assistant2::Submit", "Codebase"]
+      "cmd-enter": ["assistant2::Submit", "Codebase"],
+      "escape": "assistant2::Cancel"
     }
   },
   {