editor: Add action to move between snippet tabstop positions (#41466)

Closes #41407

This solves a problem where users couldn't navigate between snippet
tabstops while the completion menu was open.

I named the action {Next, Previous}SnippetTabstop instead of Placeholder
to be more inline with the LSP spec naming convention and our codebase
names.

Release Notes:

- Editor: Add actions to move between snippet tabstop positions

crates/editor/src/editor.rs

@@ -2439,6 +2439,10 @@ impl Editor {
             key_context.add("renaming");
         }
 
+        if !self.snippet_stack.is_empty() {
+            key_context.add("in_snippet");
+        }
+
         match self.context_menu.borrow().as_ref() {
             Some(CodeContextMenu::Completions(menu)) => {
                 if menu.visible() {
@@ -9947,6 +9951,38 @@ impl Editor {
         self.outdent(&Outdent, window, cx);
     }
 
+    pub fn next_snippet_tabstop(
+        &mut self,
+        _: &NextSnippetTabstop,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if self.mode.is_single_line() || self.snippet_stack.is_empty() {
+            return;
+        }
+
+        if self.move_to_next_snippet_tabstop(window, cx) {
+            self.hide_mouse_cursor(HideMouseCursorOrigin::TypingAction, cx);
+            return;
+        }
+    }
+
+    pub fn previous_snippet_tabstop(
+        &mut self,
+        _: &PreviousSnippetTabstop,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if self.mode.is_single_line() || self.snippet_stack.is_empty() {
+            return;
+        }
+
+        if self.move_to_prev_snippet_tabstop(window, cx) {
+            self.hide_mouse_cursor(HideMouseCursorOrigin::TypingAction, cx);
+            return;
+        }
+    }
+
     pub fn tab(&mut self, _: &Tab, window: &mut Window, cx: &mut Context<Self>) {
         if self.mode.is_single_line() {
             cx.propagate();