editor: Add action to move between snippet tabstop positions v2 (#42127)

Closes https://github.com/zed-industries/zed/issues/41407

This PR fixes the issues that caused #41407 to be reverted in #42008.
Namely that the action context didn't take into account if a snippet
could move backwards or forwards, and the action shared the same key
mapping as `editor::MoveToPreviousWordStart` and
`editor::MoveToNextWordEnd`.

I changed the default key mapping for the move to snippet tabstop to tab
and shift-tab to match the default behavior of other editors.

Release Notes:

- Editor: Add actions to move between snippet tabstop positions

crates/editor/src/editor.rs

@@ -2511,6 +2511,18 @@ impl Editor {
             key_context.add("renaming");
         }
 
+        if let Some(snippet_stack) = self.snippet_stack.last() {
+            key_context.add("in_snippet");
+
+            if snippet_stack.active_index > 0 {
+                key_context.add("has_previous_tabstop");
+            }
+
+            if snippet_stack.active_index < snippet_stack.ranges.len().saturating_sub(1) {
+                key_context.add("has_next_tabstop");
+            }
+        }
+
         match self.context_menu.borrow().as_ref() {
             Some(CodeContextMenu::Completions(menu)) => {
                 if menu.visible() {
@@ -10046,6 +10058,42 @@ 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() {
+            cx.propagate();
+            return;
+        }
+
+        if self.move_to_next_snippet_tabstop(window, cx) {
+            self.hide_mouse_cursor(HideMouseCursorOrigin::TypingAction, cx);
+            return;
+        }
+        cx.propagate();
+    }
+
+    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() {
+            cx.propagate();
+            return;
+        }
+
+        if self.move_to_prev_snippet_tabstop(window, cx) {
+            self.hide_mouse_cursor(HideMouseCursorOrigin::TypingAction, cx);
+            return;
+        }
+        cx.propagate();
+    }
+
     pub fn tab(&mut self, _: &Tab, window: &mut Window, cx: &mut Context<Self>) {
         if self.mode.is_single_line() {
             cx.propagate();