feat: add includeMathExtensions option to createMarkdownToReactParser

[wendyyuchensun]

Summary

• Adds includeMathExtensions option to createMarkdownToReactParser (default: true)
• When false, the math and nemeth marked extensions are excluded from the parser — useful for consumers who don't need math rendering and want a lighter parse
• Default behavior is unchanged: math (LaTeX/AsciiMath) and Nemeth extensions are included as before
• Documents the new option in README

Usage

To opt out of math/Nemeth rendering:

createMarkdownToReactParser({
  options: {
    includeMathExtensions: false,
    // ...other options
  },
  components,
})