Add VS Code extension for Sidemantic

[nicosuave]

Adds a VS Code extension that provides:

• Syntax highlighting for .sidemantic.sql files
• LSP integration (completions, hover, diagnostics) via sidemantic lsp
• YAML schema validation via Red Hat YAML extension

Requires uv pip install sidemantic[lsp] for LSP features.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, you can upgrade your account or add credits to your account and enable them for code reviews in your settings.