fix: skip regex checks on autolink URLs

[Kubudak90]

Fixes #91.

Autolinks like <https://example.com/eip7002> have their URL text incorrectly checked against regex patterns, causing false positives when URLs contain prohibited patterns (e.g. eip7002 matching the markdown-re-eip-dash regex).

Root Cause

The visitor pattern traverses all text nodes, including those inside autolinks. For autolinks, the URL itself becomes the text content and gets checked by enter_text, which shouldn't happen.

Solution

Use a stack-based approach to track link URLs:

1. When entering a link, push its URL onto a stack
2. When entering text, if text equals the top URL, it's an autolink — skip the pattern check
3. When departing a link, pop the URL from the stack

This correctly handles:

• Autolinks: <url> — URL text is not checked
• Regular links: [text](url) — link text is still checked
• Nested links: Stack handles nesting correctly

Testing

All existing tests pass, plus new tests verify:

• Autolink URLs don't trigger false positives
• Regular link text is still properly checked