Fix KaTeX equation numbering in notebook preview

[pingren]

Fixes #155888

Explanation

the KaTex has a stylesheet

https://github.com/KaTeX/KaTeX/blob/9164a15ef817161aa8f33beb0688e660fff87dd2/src/katex.less#L677

body {
	counter-reset: katexEqnNo mmlEqnNo;
}

which will compile into vscode/extensions/markdown-math/notebook-out/katex.min.css

However, the body style is not effective inside shadow-root element and will cause #155888.

Adding this styles to shadow root explicitly will fix the issue.

Before:

After:

[mjbvz]

Thanks!