[Feature]: Enable katex for math formula

[zhoubin-me]

I have updated mdsvex.config.js as like:

import { defineMDSveXConfig as defineConf } from 'mdsvex';
import rehypeKatexSvelte from 'rehype-katex-svelte';
import remarkMath from 'remark-math';
...

const config = defineConf({
	remarkPlugins: [remarkMath],
	rehypePlugins: [
		[
			rehypeKatexSvelte,
			{
				output: 'mathml'
			}
		]
	]
});

But still cannot render math correctly, any clue?