Variable Fonts Support for Custom Packager

[ayuhito]

Description

The current custom packager only supports non-variable fonts. I think having the option to generate a variable package would be great as variable fonts become more and more popular.

[herrstrietzel]

Thanks for this great project!

As a starting point you might include the axes data in the API item properties similar to the official google API output:

Example: Open Sans

https://api.fontsource.org/v1/fonts?family=Open+Sans&variable=true

[
    {
        "id": "open-sans",
        "family": "Open Sans",
        "subsets": [
            "cyrillic",
            "cyrillic-ext",
            "greek",
            "greek-ext",
            "hebrew",
            "latin",
            "latin-ext",
            "vietnamese"
        ],
        "weights": [
            300,
            400,
            500,
            600,
            700,
            800
        ],
        "styles": [
            "italic",
            "normal"
        ],

        "axes": [
            {
              "tag": "wdth",
              "start": 75,
              "end": 100
            },
            {
              "tag": "wght",
              "start": 300,
              "end": 800
            }
          ],
        "defSubset": "latin",
        "variable": true,
        "lastModified": "2022-09-22",
        "category": "sans-serif",
        "license": "OFL-1.1",
        "type": "google"
    }
]

[ayuhito]

The actual complexity of this lies in the subsetting of these fonts. We want to ship individual axes as well as custom mixed axes such as standard, so it would be likely for us to need to integrate fonttools/fontmake ourselves extensively for processing.

This honestly seems like a lot of work to support and maintain. Since Google Fonts, which has a full fledged team, is already supporting most of this, it also seems unnecessary for us to replicate their build pipeline and instead wait for them to add new fonts.

If there's a simpler solution that can be easily automated, I'd still be open to it.