Variable font axis setting support

[ctrlcctrlv]

Some modern programming fonts, such as JetBrains Mono, come in variable font format. See https://github.com/JetBrains/JetBrainsMono/tree/master/fonts/variable.

This is cool because it allows fine grained control of how bold you want your bold to be, beyond distributed masters.

Incosolata now also has a variable font. One of my favorite things about it is that it also has a width axis, so you can make bold wider as well if you wish, or make both wider.

Would you accept a patch in the genre of #2248 to allow setting variable font axes when specifying font_family etc? This would make Kitty very modern indeed.

[kovidgoyal]

In principle, yes.