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/diagnostics.rs

@@ -0,0 +1,104 @@
+//! Diagnostic messages produced while parsing configuration / CLI args.
+
+use std::fmt;
+
+/// A diagnostic message from parsing. This is used to provide additional
+/// human-friendly warnings and errors about the parsed data.
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct Diagnostic {
+    pub location: Location,
+    pub key: String,
+    pub message: String,
+}
+
+impl Diagnostic {
+    /// Write the full user-friendly diagnostic message to the writer.
+    pub fn format(&self, writer: &mut dyn fmt::Write) -> fmt::Result {
+        match &self.location {
+            Location::None => {}
+            Location::Cli(index) => write!(writer, "cli:{index}:")?,
+            Location::File { path, line } => write!(writer, "{path}:{line}:")?,
+        }
+
+        if !self.key.is_empty() {
+            write!(writer, "{}: ", self.key)?;
+        } else if self.location != Location::None {
+            write!(writer, " ")?;
+        }
+
+        write!(writer, "{}", self.message)
+    }
+}
+
+impl fmt::Display for Diagnostic {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.format(f)
+    }
+}
+
+/// The possible locations for a diagnostic message. This is used
+/// to provide context for the message.
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub enum Location {
+    #[default]
+    None,
+    Cli(usize),
+    File {
+        path: String,
+        line: usize,
+    },
+}
+
+/// The tag of a [`Location`] (Zig: `Location.Key`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum LocationKey {
+    None,
+    Cli,
+    File,
+}
+
+impl Location {
+    #[must_use]
+    pub fn key(&self) -> LocationKey {
+        match self {
+            Location::None => LocationKey::None,
+            Location::Cli(_) => LocationKey::Cli,
+            Location::File { .. } => LocationKey::File,
+        }
+    }
+}
+
+/// A list of diagnostics.
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct DiagnosticList {
+    list: Vec<Diagnostic>,
+}
+
+impl DiagnosticList {
+    pub fn append(&mut self, diag: Diagnostic) {
+        self.list.push(diag);
+    }
+
+    #[must_use]
+    pub fn is_empty(&self) -> bool {
+        self.list.is_empty()
+    }
+
+    /// Zig: `empty()`.
+    #[must_use]
+    pub fn empty(&self) -> bool {
+        self.list.is_empty()
+    }
+
+    #[must_use]
+    pub fn items(&self) -> &[Diagnostic] {
+        &self.list
+    }
+
+    /// Returns true if there are any diagnostics for the given
+    /// location type.
+    #[must_use]
+    pub fn contains_location(&self, location: LocationKey) -> bool {
+        self.list.iter().any(|d| d.location.key() == location)
+    }
+}