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.
