[2.x] Normalize Markdown heading identifiers

[emmadesilva]

• Follow up to [2.x] Custom Markdown heading renderer #2047
• Targets [2.x] New asset system #2006

This pull request modifies the heading identifier generation to improve how heading slugs/IDs are created. The key changes include:

1. Replacing the simple Str::slug() method with a more robust makeIdentifier() method in the HeadingRenderer class.

2. Adding more comprehensive normalization of heading text:

   • Transliterate non-ASCII characters
   • Replace certain special characters (e.g., '&' becomes 'and', '<' and '>' are removed)
   • Handle emojis by removing them
   • Normalize accented characters
   • Trim and clean up various edge cases

3. Ensure the identifier works consistently across different types of input, including:

   • Special characters
   • Emojis
   • Accented characters
   • Non-Latin scripts
   • Headings with leading/trailing spaces

4. Added comprehensive unit tests to verify the new identifier generation works correctly for a wide range of input scenarios.

The goal is creating more predictable and clean URL-friendly identifiers for headings in markdown documents, while maintaining readability and handling internationalization.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3cc57cb) to head (50f89f8).
Report is 24 commits behind head on new-asset-system.

Additional details and impacted files

@@                 Coverage Diff                  @@
##             new-asset-system     #2059   +/-   ##
====================================================
  Coverage              100.00%   100.00%           
- Complexity               1931      1932    +1     
====================================================
  Files                     196       196           
  Lines                    5139      5141    +2     
====================================================
+ Hits                     5139      5141    +2     

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