[codex] Add VS Code ggsql plot preview

[dataders]

Summary

• add a Render Plot editor toolbar command for ggsql files
• run the ggsql CLI to generate Vega-Lite JSON and open a reusable Markdown-preview-style webview
• bundle a CSP-safe local Vega renderer, responsive chart sizing, CLI availability prompts, and output-pane diagnostics
• keep upstream ggsql cell/code-lens commands from main

Screenshot

Screenshot from local testing should be embedded here once uploaded as a GitHub asset.

Validation

• npm test
• npm run compile
• npm run lint