WidthCache is added document.body, it should be a child of .xterm-helpers ideally

This could cause issues depending on styles differ between the terminal element and `body`

https://github.com/xtermjs/xterm.js/blob/32c616512d428cb392badfcde7d7ee69b37bfb94/src/browser/renderer/dom/WidthCache.ts#L77