Integrate kb_text_shape for ligatures

[david-vanderson]

Fixes #662

This is really rough so far, but I can run zig build sdl3-app and type tokiasoweli[_pilin_e_lili_ijo] nasinpi++ao and it almost works. Something is off at the end with those ++ things.

Got a ways to go, but this is at least promising.

Still a bunch of stuff to be figured out:

• loading Neptunian Shpre font crashes
• pressing a single enter crashes
• kb_text_shape sizing (and other metrics) don't account for hinting (how do we handle this?)
• textEntry cursor positioning and movement
• performance implications?

[MichaelBelousov]

I'd be curious about measured binary size increase, especially on the web backend