Add support for merging in ligatures from other fonts

[karambaq]

Hello, great work.
Can we add ligatures of the long arrows like this?

[vikky49]

The idea behind this project is to just patch the ligatures from Fira code not to create new ones

[karambaq]

ok, just ask how can I do this

[ToxicFrog]

You'll need to either find a font that has a ligature for -----> or create one yourself using a font editor like Fontforge. Once you've done that you can use the same techniques ligaturizer uses to splice it into your font; I'd be open to adding features to ligaturizer that would let it pull ligatures from multiple sources, if it doesn't turn out to be too complicated.

[karambaq]

@ToxicFrog I think Iosevka has this.
https://github.com/be5invis/Iosevka