Use a HarfBuzz based text system

[mikayla-maki]

Our current text system:

• Doesn't have full support for font features (Can't set font features and variations pop-os/cosmic-text#229)
• Doesn't use fontconfig
• Has some duplicated caching between the GPUI and the platform text system.
• Doesn't provide full font metrics

Let's replace it with HarfBuzz

[jackpot51]

Please consider working with me on cosmic-text support for any features you need. If you directly use harfbuzz you will need to implement your own layout and font fallback code, which is not trivial.

[panekj]

• Doesn't use fontconfig

This seems to be either untrue or an oversight in your usage of cosmic-text as it provides fontconfig feature that is enabled by default, it's backed by fontdb and not fontconfig library (so it doesn't link to it), doesn't support all of fontconfig, but some subset of it is supported.

[Speykious]

I'm reading that cosmic-text is more likely to continue being used here, but if you're considering harfbuzz, consider also using swash instead. There's also a complex demo of it (for context, this is a fork of mine that is more up-to-date).