Comment support for katex

[nmanumr]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Affected packages and versions

rehype-katex@6.0.0

Link to runnable example

No response

Steps to reproduce

try the following equation in the markdown

$$
\begin{split}
   f(-2) &= \sqrt{-2+4} \\
   &= x % Test Comment
\end{split}
$$

with following processors

let processor = unified()
    .use(markdown)
    .use(math)
    .use(remark2rehype)
    .use(katex)
    .use(document, {css: ['https://cdn.jsdelivr.net/npm/katex@0.13.13/dist/katex.min.css']})
    .use(html);

Expected behavior

Comments should work properly

Actual behavior

The following warnings appear in the console:

LaTeX-incompatible input and strict mode is set to 'warn': % comment has no terminating newline; LaTeX would fail because of commenting the end of math mode (e.g. $) [commentAtEnd]
docs/maths-1/ch1/ex1.1/q1-i.md
  3:1-8:3  warning  KaTeX parse error: Expected & or \\ or \cr or \end at end of input: …ent \end{split}  parseerror  rehype-katex

Runtime

Node v14

Package manager

No response

OS

macOS

Build and bundle tools

Other (please specify in steps to reproduce)

[nmanumr]

In reality, newlines are being ignored in by toText method here: https://github.com/remarkjs/remark-math/blob/main/packages/rehype-katex/index.js#L42

If you use hast-util-to-string instead of hast-util-to-text the problem gets resolved.