feat(rule): link check to include frontmatter links

[jackchuka]

This pull request enhances the link validation logic to support and validate links specified within frontmatter metadata in Markdown documents. The changes ensure that URLs and internal anchors in frontmatter are subject to the same validation rules (blocked/allowed domains, internal anchor checks) as those in the main content. It also adds comprehensive tests to cover these scenarios.

Frontmatter Link Extraction and Validation:

• Added a new function extractFrontmatterLinks in extractor.go to extract link-like values (URLs and anchors) from frontmatter data, storing them in the Links field of the FrontMatter struct. [1] [2]
• Modified the parser (parser.go) to populate the new Links field in the FrontMatter struct during document parsing.
• Updated the link validation rule (link.go) to include links from frontmatter in its validation checks.

Testing Enhancements:

• Added a suite of tests in link_test.go to verify validation of frontmatter links, including checks for blocked/allowed domains, internal anchors, and non-string values.
• Added a new integration test case and test data for a document with a blocked domain in frontmatter. [1] [2]