Implement regex_select action for Helix (#38736)

Closes #31561

Release Notes:

- Implemented the select_regex Helix keymap

Prior: The keymap `s` defaulted to `vim::Substitute`

After:
<img width="1387" height="376" alt="image"
src="https://github.com/user-attachments/assets/4d3181d9-9d3f-40d2-890f-022655c77577"
/>

Thank you to @ConradIrwin for pairing to work on this

crates/vim/src/normal/search.rs

@@ -195,6 +195,7 @@ impl Vim {
                         prior_selections,
                         prior_operator: self.operator_stack.last().cloned(),
                         prior_mode,
+                        helix_select: false,
                     }
                 });
             }
@@ -218,6 +219,12 @@ impl Vim {
         let new_selections = self.editor_selections(window, cx);
         let result = pane.update(cx, |pane, cx| {
             let search_bar = pane.toolbar().read(cx).item_of_type::<BufferSearchBar>()?;
+            if self.search.helix_select {
+                search_bar.update(cx, |search_bar, cx| {
+                    search_bar.select_all_matches(&Default::default(), window, cx)
+                });
+                return None;
+            }
             search_bar.update(cx, |search_bar, cx| {
                 let mut count = self.search.count;
                 let direction = self.search.direction;