feat: allow passing range for custom font-weight

[vojty]

This PR allows defining a range for font-weight when using a custom font face.
Eg.

    Unfonts({
      custom: {
        preload: true,
        families: {
          'Source Sans 3': {
            src: './src/assets/fonts/source-sans/SourceSans3VF-Upright.otf.woff2',
            transform: (font) => {
              font.weight = '200 900';
              return font;
            },
          },
        },
      },
    }),

[vojty]

@stafyniaksacha hey can you please take a look?