Problem:
The builtin terminfo defs don't include xterm-ghostty, so features like
`kTerm_set_underline_style` are missing when building without unibilium.
Solution:
- Add ghostty to `gen_terminfo.lua`.
- Note: The ncurses defs are somewhat different than what ghostty ships.
- Special-case ghostty in `terminfo_from_builtin`.
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