Enhance deno check --doc to support Markdown Code Blocks

[EGAMAGZ]

Currently, deno check --doc focuses on verifying inline documentation comments (JSDoc) for correctness. However, code examples embedded in Markdown files are not type-checked. This can result in discrepancies between documented examples and actual source code behavior, leading to confusion for developers relying on documentation.

Proposed Solution:

• Extend deno check --doc to:
• Parse Markdown files (e.g., README.md).
• Identify code blocks explicitly marked with language identifiers (ts, js, tsx, jsx).
• Type-check these code blocks using the Deno runtime type system.
• Report errors for mismatches, invalid syntax, or type issues.

Usage Example

deno check --doc README.md

Expected Behavior:

• Valid code examples pass without errors.
• Invalid examples generate meaningful type-check error messages.