onboarding: Wire up tab index (#35659)

Closes #ISSUE

Allows tabbing through everything in all three pages. Until #35075 is
merged it is not possible to actually "click" tab focused buttons with
the keyboard.

Additionally adds an action `onboarding::Finish` and displays the
keybind. The action corresponds to both the "Skip all" and "Start
Building" buttons, with the keybind displayed similar to how it is for
the page nav buttons

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: MrSubidubi <finn@zed.dev>

assets/keymaps/default-linux.json

@@ -1175,7 +1175,8 @@
     "bindings": {
       "ctrl-1": "onboarding::ActivateBasicsPage",
       "ctrl-2": "onboarding::ActivateEditingPage",
-      "ctrl-3": "onboarding::ActivateAISetupPage"
+      "ctrl-3": "onboarding::ActivateAISetupPage",
+      "ctrl-escape": "onboarding::Finish"
     }
   }
 ]

assets/keymaps/default-macos.json

@@ -1277,7 +1277,8 @@
     "bindings": {
       "cmd-1": "onboarding::ActivateBasicsPage",
       "cmd-2": "onboarding::ActivateEditingPage",
-      "cmd-3": "onboarding::ActivateAISetupPage"
+      "cmd-3": "onboarding::ActivateAISetupPage",
+      "cmd-escape": "onboarding::Finish"
     }
   }
 ]