Support default font_features from fontconfig

[Diaoul]

As discussed here: #2248

[Diaoul]

I'll do a pass on the docs when I get the validation of the implementation 📝

For reference:

cat ~/.config/fontconfig/fonts.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
  <alias>
    <family>monospace</family>
      <prefer>
        <family>Fira Code</family>
      </prefer>
  </alias>
  <match target="font">
    <test name="family">
      <string>Fira Code</string>
    </test>
    <edit name="fontfeatures" mode="append">
      <string>ss01</string>
    </edit>
  </match>
</fontconfig>

[kovidgoyal]

Looks generally fine with a quick glance over. Add the docs in config_data.py and I will review in detail.

[Diaoul]

Actually there is a bug in pattern_as_dict: it does not handle multiple values correctly. This applies to FC_FONT_FEATURES but maybe others as well. For reference, here is how multiple values are handled by FcPatternPrint using FcValueListPrint.

[kovidgoyal]

Feel free to send a separate PR to fix it or wait and I will get around
to it when I have a moment.

[kovidgoyal]

Also, what's the bug, as far as I know, patter_as_dict doesnt get any non-scalar values currently.

[Diaoul]

I just fixed it for font_features. As for the others I don't know if they are lists or not and I could not find a reliable source of information for this. Here FC_FONT_FEATURES is defined as a String...

I've seen references to fonts being able to be part of multiple families somewhere (sorry lost the reference) and despite family being singular, the description reads Font family names. 🤷‍♂️

[Diaoul]

Please doublecheck that one @kovidgoyal, I tried to make sure I've cleaned up properly but I might have missed something.