Use modern CSS normalizer

[axelboc]

https://github.com/sindresorhus/modern-normalize

• This removes a bunch of CSS declarations targeting IE and other obsolete browsers.
• The new modern-normalize.css includes its own box-sizing declaration, so I remove it from our side.
• One of the -webkit declarations fixes the focus outlines of buttons on Chromium as shown in the Cypress snapshots.

I also take the opportunity to:

• Use the :where() selector in the viewer's App.module.css where we apply our own "bare element" styles, so as to avoid increasing CSS specificity. This selector is "widely available" since 2021.
• Add a normalization style in App.module.css for pre and kbd elements. This makes the monospace font of keyboard shortcuts in the interactions help menu consistent with other monospace pre elements.