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

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

crates/ghostty-config/src/config-template

@@ -0,0 +1,52 @@
+# This is the configuration file for Ghostty.
+#
+# This template file has been automatically created at the following
+# path since Ghostty couldn't find any existing config files on your system:
+#
+#   {[path]s}
+#
+# The template does not set any default options, since Ghostty ships
+# with sensible defaults for all options.
+# Note that you should not paste the output of `ghostty +show-config
+# --default` into your config: some default options actually conflict with each other
+# when explicitly set in a configuration file. Instead, only set the
+# options you actually need.
+#
+# Run `ghostty +show-config --default --docs` to view a list of
+# all available config options and their default values.
+#
+# Additionally, each config option is also explained in detail
+# on Ghostty's website, at https://ghostty.org/docs/config.
+#
+# Ghostty can reload the configuration while running by using the menu
+# options or the bound key (default: Command + Shift + comma on macOS and
+# Control + Shift + comma on other platforms). Not all config options can be
+# reloaded while running; some only apply to new windows and others may require
+# a full restart to take effect.
+
+# Config syntax crash course
+# ==========================
+# # The config file consists of simple key-value pairs,
+# # separated by equals signs.
+# font-family = Iosevka
+# window-padding-x = 2
+#
+# # Spacing around the equals sign does not matter.
+# # All of these are identical:
+# key=value
+# key= value
+# key =value
+# key = value
+#
+# # Any line beginning with a # is a comment. It's not possible to put
+# # a comment after a config option, since it would be interpreted as a
+# # part of the value. For example, this will have a value of "#123abc":
+# background = #123abc
+#
+# # Empty values are used to reset config keys to default.
+# key =
+#
+# # Some config options have unique syntaxes for their value,
+# # which is explained in the docs for that config option.
+# # Just for example:
+# resize-overlay-duration = 4s 200ms