Initial package setup #1

New package extracted from the secvisogram (see secvisogram#606) needs the following to become a proper standalone package:

• Module for enriching CSAF 2.0/2.1 documents with template data (enrichDocumentV2_0/enrichDocumentV2_1), including CVSS v2/v3/v4 support for 2.1
• Module for rendering markdown fields to HTML (renderMarkdown)
• Mustache templates + rendering functions for CSAF 2.0 and 2.1 (HTMLTemplate2_0/HTMLTemplate2_1)
• Self-contained HTML output (inline CSS, no external file/network dependencies)
• Test suite
• Prettier config + test script (type-check, format check, tests)
• package.json metadata (repository, bugs, homepage, correct files, correct license)
• CI (tests, npm publish, dependabot)
• README (usage, API, limitations)