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

assets/keymaps/default-linux.json

@@ -735,6 +735,20 @@
       "tab": "editor::ComposeCompletion"
     }
   },
+  {
+    "context": "Editor && in_snippet && has_next_tabstop && !showing_completions",
+    "use_key_equivalents": true,
+    "bindings": {
+      "tab": "editor::NextSnippetTabstop"
+    }
+  },
+  {
+    "context": "Editor && in_snippet && has_previous_tabstop && !showing_completions",
+    "use_key_equivalents": true,
+    "bindings": {
+      "shift-tab": "editor::PreviousSnippetTabstop"
+    }
+  },
   // Bindings for accepting edit predictions
   //
   // alt-l is provided as an alternative to tab/alt-tab. and will be displayed in the UI. This is