feat: add shortcode completion provider

[mcanouil]

Summary

• Add a shortcode parser (src/utils/shortcodeParser.ts) that tokenises Quarto {{< name key="value" >}} syntax and determines cursor context (name, argument, attribute key, or attribute value).
• Add a completion provider (src/providers/shortcodeCompletionProvider.ts) that discovers shortcode schemas from installed extensions via SchemaCache and offers contextual completions for shortcode names, attribute keys, and attribute values.
• Register the provider in src/extension.ts for the quarto language ID with space, =, and < as trigger characters.
• Add Mocha tests (src/test/suite/shortcodeParser.test.ts) covering bounds detection, cursor context identification, quoted/unquoted values, multiline shortcodes, and edge cases.

Test plan

• Verify shortcode parser tests pass via npm run test.
• Verify TypeScript compiles without errors via npx tsc --noEmit.
• Verify ESLint passes without warnings on changed files.
• Open a .qmd file with installed extensions that have _schema.yml containing shortcode definitions, type {{< and confirm shortcode name completions appear.
• After selecting a shortcode name, confirm attribute key completions appear.
• After typing key=, confirm attribute value completions appear for enum-typed attributes.