Add .ghostty extension to config (#8885)

Resolves #8689

For various reason, ghostty wants to have a unique file extension for
the config files. The name was settled on `config.ghostty`. This will
help with tooling. See #8438 (original discussion) for more details.

This PR introduces the preferred default of `.ghostty` while still
supporting the previous `config` file. If both files exist, a warning
log is sent.

The docs / website will need to be updated to reflect this change. 

> [!NOTE]
> Only tested on macOS 26.0.

---------

Co-authored-by: Mitchell Hashimoto <m@mitchellh.com>

src/cli/edit_config.zig

@@ -30,9 +30,9 @@ pub const Options = struct {
 /// this yet.
 ///
 /// The filepath opened is the default user-specific configuration
-/// file, which is typically located at `$XDG_CONFIG_HOME/ghostty/config`.
+/// file, which is typically located at `$XDG_CONFIG_HOME/ghostty/config.ghostty`.
 /// On macOS, this may also be located at
-/// `~/Library/Application Support/com.mitchellh.ghostty/config`.
+/// `~/Library/Application Support/com.mitchellh.ghostty/config.ghostty`.
 /// On macOS, whichever path exists and is non-empty will be prioritized,
 /// prioritizing the Application Support directory if neither are
 /// non-empty.
@@ -73,7 +73,7 @@ fn runInner(alloc: Allocator, stderr: *std.Io.Writer) !u8 {
     defer config.deinit();
 
     // Find the preferred path.
-    const path = try Config.preferredDefaultFilePath(alloc);
+    const path = try configpkg.preferredDefaultFilePath(alloc);
     defer alloc.free(path);
 
     // We don't currently support Windows because we use the exec syscall.