feat(dist): generate completions, editor syntax, man pages, translations

Ports Ghostty's build-data helpers (main_build_data.zig, src/extra/*,
build/mdgen) as the ghostty-build-data binary: bash/zsh/fish
completions, Vim syntax/ftdetect/ftplugin/compiler, Sublime syntax (also
used by bat), terminfo source and ghostty(1)/ghostty(5) markdown. With
the version set to the installed 1.3.1-arch2, all generated files are
byte-identical to the installed Ghostty's.

- `just docs` renders man pages and HTML with pandoc.
- po/: Ghostty's 29 translation catalogs; `just locales` compiles them
  into the com.mitchellh.ghostty gettext domain.
- `just install PREFIX FLATPAK SYSTEM_PACKAGE` installs everything incl.
  terminfo text/termcap outputs and mirrors GhosttyResources.zig install
  modes (Flatpak D-Bus template without systemd; lib/systemd/user for
  system packages).
- Build metadata (version, tip/stable channel, commit, build mode) is
  derived at build time like build_config.zig; GTK/libadwaita
  compile-time versions come from pkg-config instead of constants.

Test Plan:
- cargo run -p ghostty-build-data -- +bash | diff - \
  /usr/share/bash-completion/completions/ghostty (same for zsh/fish/vim)
- just install /tmp/prefix

crates/ghostty-build-data/src/vim.rs

@@ -0,0 +1,20 @@
+//! Vim/Neovim plugin files (port of `extra/vim.zig`).
+
+use std::fmt::Write as _;
+
+use crate::chunks_vim::{C0, C1, C2, C3, C4};
+use crate::config_fields;
+
+pub const FTDETECT: &str = C0;
+pub const FTPLUGIN: &str = C1;
+pub const COMPILER: &str = C2;
+
+#[must_use]
+pub fn syntax() -> String {
+    let mut w = String::from(C3);
+    for f in config_fields() {
+        let _ = write!(w, "\n\t\\ {}", f.name);
+    }
+    w.push_str(C4);
+    w
+}