Add HarfBuzz for text shaping (complex scripts, font ligatures)

[bruvzg]

Change DynamicFont to use HarfBuzz (MIT license) to handle:

1. Context-sensitive shaping and ligatures for complex scripts (Arabic, Devanagari, Thai etc.);
2. Programming ligatures with fonts like Iosevka & Fira Code in script editor.

Related: #3081

[Juan Linietsky]

Would be ideal to just adapt the relevant code instead of using a pretty big library for it

[bruvzg]

This is far from finished and a hell of a mess, but here's early in-progress screenshot (bidi display, language specific shaping, OT typographic features).

Test build:

3.0 alpha 1:

I'll probably make WIP PR next week.

[Rémi Verschelde]

Superseded by godotengine/godot-proposals#4.