Support MathML (mml:*) elements for XHTML rendering

[lacatoire]

Summary

• Add mappings for all presentation MathML elements (mml:math, mml:mi, mml:mn, mml:mo, …) in
  the Generic XHTML format
• A single format_mml_element() handler strips the mml: prefix and outputs standard HTML5 MathML
  tags (<math>, <mi>, etc.)
• Adds xmlns="http://www.w3.org/1998/Math/MathML" on the <math> root element for XHTML
  compatibility
• Preserves MathML attributes (display, mathvariant, width, …) and xml:id → id
• Handles self-closing elements (e.g. <mspace/>)
• Does not inject class= attributes (unlike transformFromMap)
• Text nodes inside MathML elements are handled by the existing TEXT() method

Closes #172

Test plan

• New .phpt test covering inline equation, self-closing <mspace/>, and mathvariant
  attribute
• Full existing test suite still passes

[Girgias]

Thank you for working on this!