Make cmd-t open new tab in Assistant (#25267)

Match the behavior of chrome, safari, etc.

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -185,6 +185,7 @@
       "ctrl-k h": "assistant::DeployHistory",
       "ctrl-k l": "assistant::DeployPromptLibrary",
       "new": "assistant::NewContext",
+      "ctrl-t": "assistant::NewContext",
       "ctrl-n": "assistant::NewContext"
     }
   },

assets/keymaps/default-macos.json

@@ -209,6 +209,7 @@
       "cmd-alt-/": "assistant::ToggleModelSelector",
       "cmd-k h": "assistant::DeployHistory",
       "cmd-k l": "assistant::DeployPromptLibrary",
+      "cmd-t": "assistant::NewContext",
       "cmd-n": "assistant::NewContext"
     }
   },