Codebase indexing should be supported as it is in the Vscode extension

### Describe the feature or problem you'd like to solve

Codebase Indexing https://docs.github.com/en/copilot/concepts/context/repository-indexing

### Proposed solution

Overall there should be feature parity with the VS Code extension but at a minimum the codebase indexing should be supported first

This enables working with large codebases without friction

### Example prompts or workflows

_No response_

### Additional context

_No response_