Distribute a 'duotone' font for coding

It would be nice to render italics in a more 'cursive' font than non-italics to make code easier to read. While VSCode doesn't have native support for rendering multiple fonts, [Recursive Mono](https://www.recursive.design/) achieves it anyway by distributing a "Rec Mono Duotone" font that replaces the italic forms with a more cursive variant (what they call "casual") of the font:

Setting the font to "Monaspace Argon" on the other hand results in italics that look more like regular letters but slanted (with a few exceptions like the `f`):

This makes the italics harder to distinguish, making code more difficult to read. It would be really cool to try a Argon or Neon but with the italic glyphs replaced with Radon or Xenon.

Start by reviewing the existing Monaspace Argon, Neon, Radon, and Xenon font designs and compare them with Recursive Mono Duotone. Determine how a duotone variant could replace italic glyphs with a more cursive companion, then verify that the resulting font can be distributed and selected as a usable coding font.

索引モデルが issue の本文から書いたものです。