[fontless] support selecting preloaded fonts by subsets

[hi-ogawa]

🆒 Your use case

With #648, it supports adding preload links to html, but I noticed the current preload font selection based on meta.priority has an issue since there are often multiple fonts with meta.priority: 0. In the example added in #648, it shows "latin" subset of "Poppins" font is not preloaded and it's still downloaded through css, which causes network waterfall.

🆕 The solution you'd like

Fontless should support selecting preload fonts based on subsets by extending current preload: boolean type to preload: boolean | string[].

fontless({
  defaults: {
    preload: ["latin"], // default
  },
  families: [
    { name: "Inter", preload: ["latin"] }, // override per font
  ]
})

Currently subset information is not available. I made a PR on unifont to expose meta.subset for this purpose unjs/unifont#248

🔍 Alternatives you've considered

No response

ℹ️ Additional info

Next.js and Astro uses subsets as an only way to select preload fonts:

• https://nextjs.org/docs/app/api-reference/components/font#subsets

import { Inter } from 'next/font/google'
 
const inter = Inter({
  subsets: ['latin'],
  display: 'swap',
})

• https://docs.astro.build/en/reference/experimental-flags/fonts/#subsets

import { defineConfig, fontProviders } from "astro/config";

export default defineConfig({
  experimental: {
    fonts: [
      {
        name: "Inter",
        cssVariable: "--font-inter",
        provider: fontProviders.fontsource(),
        ...
        subsets: ["cyrillic"],
      },