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>

crates/ai_onboarding/src/ai_upsell_card.rs

@@ -12,6 +12,7 @@ pub struct AiUpsellCard {
     pub sign_in_status: SignInStatus,
     pub sign_in: Arc<dyn Fn(&mut Window, &mut App)>,
     pub user_plan: Option<Plan>,
+    pub tab_index: Option<isize>,
 }
 
 impl AiUpsellCard {
@@ -28,6 +29,7 @@ impl AiUpsellCard {
                 })
                 .detach_and_log_err(cx);
             }),
+            tab_index: None,
         }
     }
 }
@@ -112,7 +114,8 @@ impl RenderOnce for AiUpsellCard {
                         .on_click(move |_, _window, cx| {
                             telemetry::event!("Start Trial Clicked", state = "post-sign-in");
                             cx.open_url(&zed_urls::start_trial_url(cx))
-                        }),
+                        })
+                        .when_some(self.tab_index, |this, tab_index| this.tab_index(tab_index)),
                 )
                 .child(
                     Label::new("No credit card required")
@@ -123,6 +126,7 @@ impl RenderOnce for AiUpsellCard {
             _ => Button::new("sign_in", "Sign In")
                 .full_width()
                 .style(ButtonStyle::Tinted(ui::TintColor::Accent))
+                .when_some(self.tab_index, |this, tab_index| this.tab_index(tab_index))
                 .on_click({
                     let callback = self.sign_in.clone();
                     move |_, window, cx| {
@@ -193,6 +197,7 @@ impl Component for AiUpsellCard {
                             sign_in_status: SignInStatus::SignedOut,
                             sign_in: Arc::new(|_, _| {}),
                             user_plan: None,
+                            tab_index: Some(0),
                         }
                         .into_any_element(),
                     ),
@@ -202,6 +207,7 @@ impl Component for AiUpsellCard {
                             sign_in_status: SignInStatus::SignedIn,
                             sign_in: Arc::new(|_, _| {}),
                             user_plan: None,
+                            tab_index: Some(1),
                         }
                         .into_any_element(),
                     ),