feat: add Quarto Wizard schema and snippets

[mcanouil]

This PR improves Quarto Wizard support (VS Code/Positron) for this extension by adding extension metadata files used for editor assistance.

What this adds

• _schema.yml with extension-specific fields (options/classes/attributes and/or format keys) to enable better completion, hover docs, and validation.
• _snippets.json with practical insertion snippets for common extension usage.

Why

These files improve authoring UX in Quarto projects by making extension configuration and usage easier to discover and less error-prone in VSCode/Positron when using Quarto Wizard.

References

• Schema specification: https://m.canouil.dev/quarto-wizard/reference/schema-specification.html
• Snippet specification: https://m.canouil.dev/quarto-wizard/reference/snippet-specification.html

[mcanouil]

For Quarto Wizard integrations, schema options should be scoped under extensions.<extension-name> in _schema.yml.

This scoping helps suggestions, completions, and hovering trigger reliably, and avoids option conflicts with other extensions and Quarto CLI options.

Reference: quarto-dev/quarto-cli#12894