The default slugification function processes anchor links (hash fragments). If an anchor starts with a number (e.g., #123), it is replaced with #_number (e.g., #_123). This rule is designed to prevent potential URL conflicts, though manually written anchors like #123 are rare in Markdown—hence the special handling.

Key Behaviors:
​Digits-Only Anchors:
#123 → #_123 (Adds _ to numeric anchors)
​Alphanumeric Anchors:
#123a → #_123a (Only the leading digits are modified)
​Non-Numeric Anchors:
#section → #section (No changes, letters remain untouched)
Why It Matters:
​URL Safety: Ensures numeric anchors (e.g., #1) don’t break routing in frameworks like Vue Router or static site generators.
​Markdown Context: Most anchors are auto-generated from headings (e.g., #hello-world), but explicit #123 anchors might appear in rare cases.

# 1. Test Anchor

[Link to section](#123)      → Rendered as: `<a href="#_123">Link to section</a>`  
[Link to subsection](#123a) → Rendered as: `<a href="#_123a">Link to subsection</a>`  
[Link to FAQ](#a)           → Rendered as: `<a href="#a">Link to FAQ</a>`  
[Link to Test Anchor](#1-test-anchor)           → Rendered as: `<a href="#_1-test-anchor">Link to Test Anchor</a>`