feat(math): implement m:m matrix converter

[Abdeltoto]

Closes #2601

Summary

• Implements the m:m OMML-to-MathML converter for matrices
• Maps m:m to <mtable> with <mtr> rows and <mtd> cells
• Each m:mr row's m:e cells are converted independently using convertChildren
• Returns null for empty matrices
• Registers the converter in MATH_OBJECT_REGISTRY
• Adds 3 unit tests covering 2x2 matrix, empty matrix, and 1x3 row vector

Spec reference

ECMA-376 Section 22.1.2.60

Test plan

• vitest run passes for omml-to-mathml.test.ts
• Linter and formatter pass (lefthook pre-commit)