✨ Tree Data Provider (Sidebar Tree View Content)

# Overview
A TreeDataProvider (Tree View API) gives extension developers the ability to add content to a sidebar tab. Many extensions, like ToDo, would be able to utilize this API.

See the [VS Code Tree Data Provider Section](https://code.visualstudio.com/api/extension-guides/tree-view#tree-data-provider) for implementation reference.

# Additional Features
It would be great if we could add a filter feature. This is not included in VS Code, but would be really helpful for extensions like the Tailwind CSS extension I built for Nova that I would like to port over as an early extension. Maybe we could add a keywords property as an array to the TreeItem class. The filtering could then be done using those values.

# Resources
- [VS Code Tree View API Guide](https://code.visualstudio.com/api/extension-guides/tree-view)
- [VS Code Tree View Extension Sample](https://github.com/microsoft/vscode-extension-samples/tree/main/tree-view-sample)