Ligatures support?

[fabiospampinato]

I've parsed a font that contains custom ligatures with this library, but inspecting the returned objected I don't see references to ligature mappings 🤔 So are ligatures supported? If not, could support for them be added? I'd like to use this library to generate icon fonts, but an icon font without a ligature is not as nice.

[fabiospampinato]

@kekee000 do you know very roughly when this might get implemented? I have a use case where I'd like to be able to generate an icon font with ligatures directly in the browser, and fonteditor-core seems the most promising library for something like that.

Looking at the manual it seems like one would need to support the morx table (not the mort one that seems kinda deprecated), and define a "Ligature subtable" in it, but it looks fairly complicated, at least for me that I'm unfamiliar with TTF 🤔

[fabiospampinato]

Apparently the table to implement is the gsub table, which I don't even see mentioned in Apple's TTF manual 🙃

This PR seems to be adding support for ligatures to svg2ttf, so it's probably a pretty useful reference to have.