consider enabling fontconfig feature by default

[urlordjames]

I think it's worth considering enabling the fontconfig feature be enabled by default. Without the fontconfig feature enabled, fontdb is unable to discover system fonts on NixOS (#20). Enabling the fontconfig feature by default would make dependents easier to work with on NixOS, and would prevent opening an issue to enable the feature for each dependent that wishes to support NixOS (see pop-os/cosmic-text#167).

As an alternative to enabling the fontconfig feature by default, a note could be added to README.md to inform users that enabling the feature is more portable.

Any thoughts on this?

[RazrFalcon]

The current fontconfig support isn't perfect, therefore it isn't enabled by default.
I'm not sure what would be the right move here. If it required by NixOS, then I guess we should comply with "sane defaults". But the non-fontconfig version should typically be better/faster on normal distros.

I will think about it.

PS: I'm not sure how cross-platform cosmic-text is, but a proper Linux solution would require using fontconfig directly, avoiding fontdb entirely.