Update to support latest MathJS

[mark-wiemer]

export const texToNode = (tex: string): mathjs.MathNode | null => {
  try {
    return parseTex(tex); // err: Property 'toHTML' is missing in type import(".../node_modules/tex-math-parser/node_modules/mathjs/types/index").MathNode' but required in type 'math.MathNode'.
  } catch (err) {
    return null;
  }
}