Fix incompatibility with nextjs document patching (cannot read properties of undefined, reading .style)

Nextjs patches the global `document` to make SSR and SSG work. However it does not define the `documentElement` on `document` making this line: 

https://github.com/ProseMirror/prosemirror-view/blob/83a400d0fd3702cb4819ae33e5451303909b5af4/src/browser.ts#L23

fail in nextjs dev mode.