Support font ligatures

[tizee]

close #102
Support monospace ligatures by including harfbuzz.

To render font features like ligature, it's required to use a font shaping engine. Here I use Harfbuzz for both shaping and cross-platform. So instead of writing the same logic using platform-specific shaping engine which I am also unfamiliar with, I just need to write a simple wrapper around harfbuzz-sys in hb_wrapper.rs.

Notice that I also enforce the use of freetype feature for font-kit as font-kit would use platform-specific rasterization library by default which would breaks the logic of harfbuzz.

The application works well in macOS. However, I haven't test it on Windows or Linux yet. I would give it a test on these platforms when I have time.

JetBrains Mono:

Iosevka:

Fira Code:

[Aloxaf]

Thanks! That's so cool!

Tested on Linux, works well.