vim: Add gt and gT bindings for Markdown preview mode (#39854)

### What does this PR do?
- Adds default keybindings `gt` for navigating to the next tab and `gT`
for navigating to the previous tab in markdown viewer mode

### Why do we need this change?
- While previewing markdown files, the default vim bindings (`gt` and
`gT`) do not work for navigating between tabs. These bindings work
everywhere else, which provides a non-consistent experience for the
user.

### How do we do this change?
- Update the vim mode bindings to explicitly add handling for this mode

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/vim/src/vim.rs

@@ -51,7 +51,10 @@ use vim_mode_setting::HelixModeSetting;
 use vim_mode_setting::VimModeSetting;
 use workspace::{self, Pane, Workspace};
 
-use crate::state::ReplayableAction;
+use crate::{
+    normal::{GoToPreviousTab, GoToTab},
+    state::ReplayableAction,
+};
 
 /// Number is used to manage vim's count. Pushing a digit
 /// multiplies the current value by 10 and adds the digit.
@@ -409,6 +412,46 @@ pub fn init(cx: &mut App) {
                 cx.defer_in(window, |vim, window, cx| vim.search_submit(window, cx))
             })
         });
+        workspace.register_action(|_, _: &GoToTab, window, cx| {
+            let count = Vim::take_count(cx);
+            Vim::take_forced_motion(cx);
+
+            if let Some(tab_index) = count {
+                // <count>gt goes to tab <count> (1-based).
+                let zero_based_index = tab_index.saturating_sub(1);
+                window.dispatch_action(
+                    workspace::pane::ActivateItem(zero_based_index).boxed_clone(),
+                    cx,
+                );
+            } else {
+                // If no count is provided, go to the next tab.
+                window.dispatch_action(workspace::pane::ActivateNextItem.boxed_clone(), cx);
+            }
+        });
+
+        workspace.register_action(|workspace, _: &GoToPreviousTab, window, cx| {
+            let count = Vim::take_count(cx);
+            Vim::take_forced_motion(cx);
+
+            if let Some(count) = count {
+                // gT with count goes back that many tabs with wraparound (not the same as gt!).
+                let pane = workspace.active_pane().read(cx);
+                let item_count = pane.items().count();
+                if item_count > 0 {
+                    let current_index = pane.active_item_index();
+                    let target_index = (current_index as isize - count as isize)
+                        .rem_euclid(item_count as isize)
+                        as usize;
+                    window.dispatch_action(
+                        workspace::pane::ActivateItem(target_index).boxed_clone(),
+                        cx,
+                    );
+                }
+            } else {
+                // No count provided, go to the previous tab.
+                window.dispatch_action(workspace::pane::ActivatePreviousItem.boxed_clone(), cx);
+            }
+        });
     })
     .detach();
 }