Helix Font Fallback

Helix Font Fallback computes the fallback fonts for all the fonts loaded on the page.
A fallback font is a font that most of the browsers know already which is sized to approximatively the size of the font it temporarily replaces. Like this, the browser can display nicely the page while the custom font is loaded. This is crucial to prevent CLS (Core Web Vitals).

Key features:
- computes a fallback font for all fonts loaded on the current page
- generates some CSS that can be directly copied and pasted in the existing website CSS
- allows the user to preview the fallback fonts directly in the current page