We don't really have any large outstanding regressions on -ng to warrant
keeping this alive anymore. ¡Adiós!

Fixes #2473

This commit changes `ghostty_surface_ime_point` to return a full rect
with the width/height calculated for the preedit.

The `firstRect` function, which calls `ghostty_surface_ime_point` was
previously setting the width/height to zero. macOS didn't like this. We
then changed it to just hardcode it to width/height of one cell. This
worked but made it so the IME cursor didn't follow the preedit.

First, this commit modifies libghostty to use a single unified action
dispatch system based on a tagged union versus the one-off callback
system that was previously in place. This change simplifies the code on
both the core and consumer sides of the library. Importantly, as we
introduce new actions, we can now maintain ABI compatibility so long as
our union size does not change (something I don't promise yet).

Second, this moves a lot more of the functions call on a surface into
the action system. This affects all apprts and continues the previous
work of introducing a more unified API for optional surface features.