feat(config): port the configuration system and CLI argument parser

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Ports ZIG/src/config (minus C/WASM APIs) and cli/{args,CommaSplitter,
diagnostics}.zig into ghostty-config:
- all 202 options with exact names, types, defaults and verbatim docs,
  declared through one config_struct! macro; Zig's comptime reflection is
  replaced by the ConfigField/CliTarget traits
- loading: default XDG files, -e, config-file includes (optional '?',
  cycle detection), themes incl. light:/dark: pairs, conditional config,
  replay, finalize() derived defaults, clone/changed diffing for reload
- Linux/GTK default keybinds built on ghostty_input::binding::Set
- formatter producing +show-config output

Verified against the installed Ghostty 1.3.1: `+show-config --default`
and `--default --docs` match byte for byte (except gtk-opengl-debug,
which differs between debug/release builds), as do two complex loaded
configs.

Open items (docs/rewrite/notes/ghostty-config.md): url regex needs an
Oniguruma-compatible engine (onig) in the renderer; some font/terminal
types are local copies awaiting integration; KeyRemapSet equality is
order sensitive unlike Zig.

Test Plan:
- cargo test -p ghostty-config (255 tests)
- compare `ghostty +show-config --default --docs` with our formatter

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