Performance issues with large repositories

### 🤔 What's the problem you're trying to solve?

Working with a large repository is extremely slow, often making VS Code hang. Also, on restarting VS Code it seems like the extension needs to re-find/cache the feature/step files.

### ✨ What's your proposed solution?

Perhaps a persistent cache can be used so that the indexing/caching process is done only once, unless there is a change to files/directories.

Also, if possible that process should be a separate thread so it doesn't make VS Code unresponsive.

No files, tests, or entry points are named. Start by reproducing the slowdown with a large repository and inspect the extension's feature/step indexing and caching path; done means indexing survives a VS Code restart, refreshes when files or directories change, and does not make VS Code unresponsive.

Written by the indexing model from the issue text.