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

po/README_CONTRIBUTORS.md

@@ -0,0 +1,72 @@
+# Localizing Ghostty: The Contributors' Guide
+
+Ghostty uses the `gettext` library/framework for localization, which has the
+distinct benefit of being able to be consumed directly by our two main
+app runtimes: macOS and GTK (Linux). The core would ideally remain agnostic
+to localization efforts, as not all consumers of libghostty would be interested
+in localization support. Thus, implementors of app runtimes are left responsible
+for any localization that they may add.
+
+## GTK
+
+In the GTK app runtime, translatable strings are mainly sourced from Blueprint
+files (located under `src/apprt/gtk/ui`). Blueprints have a native syntax for
+translatable strings, which look like this:
+
+```zig
+// Translators: This is the name of the button that opens the about dialog.
+title: _("About Ghostty");
+```
+
+The `// Translators:` comment provides additional context to the translator
+if the string itself is unclear as to what its purpose is or where it's located.
+
+By default identical strings are collapsed together into one translatable entry.
+To avoid this, assign a _context_ to the string:
+
+```zig
+label: C_("menu action", "Copy");
+```
+
+Translatable strings can also be sourced from Zig source files. This is useful
+when the string must be chosen dynamically at runtime, or when it requires
+additional formatting. The `i18n.` prefix is necessary as `_` is not allowed
+as a bare identifier in Zig.
+
+```zig
+const i18n = @import("i18n.zig");
+
+const text = if (awesome)
+    i18n._("My awesome label :D")
+else
+    i18n._("My not-so-awesome label :(");
+
+const label = gtk.Label.new(text);
+```
+
+All translatable strings are extracted into the _translation template file_,
+located under `po/com.mitchellh.ghostty.pot`. **This file must stay in sync with
+the list of translatable strings present in source code or Blueprints at all times.**
+A CI action would be run for every PR, which checks if the translation template
+requires any updates. You can update the translation template by running
+`zig build update-translations`, which would also synchronize translation files
+for other locales (`.po` files) to reflect the state of the template file.
+
+During the build process, each locale in `.po` files is compiled
+into binary `.mo` files, stored under `share/locale/<LOCALE>/LC_MESSAGES/com.mitchellh.ghostty.mo`.
+This can be directly accessed by `libintl`, which provide the various `gettext`
+C functions that can be called either by Zig code directly, or by the GTK builder
+(recommended).
+
+> [!NOTE]
+> For the vast majority of users, no additional library needs to be installed
+> in order to get localizations, since `libintl` is a part of the GNU C standard
+> library. For users using alternative C standard libraries like musl, they must
+> use a stub implementation such as [`gettext-tiny`](https://github.com/sabotage-linux/gettext-tiny)
+> that offer no-op symbols for the translation functions, or by using a build of
+> `libintl` that works for them.
+
+## macOS
+
+> [!NOTE]
+> The localization system is not yet implemented for macOS.