This also changes OSC strings to be null-terminated to ease lib-vt
integration. This shouldn't have any practical effect on terminal
performance, but it does lower the maximum length of OSC strings by 1
since we always reserve space for the null terminator.
Fixes various issues:
- C ABI detection was faulty, which caused some Zig programs to use
the C ABI mode and some C programs not to. Let's be explicit.
- Unit tests now tests C ABI mode.
- Build binary no longer rebuilds on any terminal change (a regression).
- Zig programs can choose to depend on the C ABI version of the terminal
lib by using the `ghostty-vt-c` module.
