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

assets/keymaps/default-linux.json

@@ -731,6 +731,14 @@
       "tab": "editor::ComposeCompletion"
     }
   },
+  {
+    "context": "Editor && in_snippet",
+    "use_key_equivalents": true,
+    "bindings": {
+      "alt-right": "editor::NextSnippetTabstop",
+      "alt-left": "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