Config: apply xdg system config files

[mjrochford]

fixes: #4506

Applies the config files inside the XDG_CONFIG_DIRS with the first file taking precedence.

[mitchellh]

Thanks for building on @jcollie's work here and keeping the commit credits. I made some notes.

[mitchellh]

Instead of all this machinery, the DirIterator could do a reverse iteration using std.mem.lastIndexOf to implement your own iterator. I would recommend this route.

[mitchellh]

Also please note my feedback on @jcollie's PR, it applies here as well.

[mjrochford]

Re. API design

Current API

os.xdg.config(alloc, .{ .subdir = "ghostty/config" });
os.xdg.cache(alloc, .{ .subdir = "ghostty/config" });

This API

To get all the xdg dirs in order from least to most important
user dirs come last after all others have been consumed

var it = os.xdg.Dir.config.iter(alloc, .{ .subdir = "ghostty/config" });

Single directory access is possible ala

os.xdg.UserDir.config.path(alloc, .{ .subdir = "ghostty" });
os.xdg.UserDir.data.path(alloc, .{ .subdir = "ghostty" });
os.xdg.UserDir.cache.path(alloc, .{ .subdir = "ghostty" });
os.xdg.UserDir.state.path(alloc, .{ .subdir = "ghostty" });

Ditto for system directories

os.xdg.SystemDir.config.iter(alloc, .{ .subdir = "ghostty" });

I've just gone ahead and used splitBackwardsScalar instead of splitScalar, I think wrapping an std iterator is better here.
Least important first makes sense to me for the Config use case, not sure about other cases.

Better API

os.programdirs.config.iter(alloc)

I think splitting XDG/Windows/MacOS known directories logic into their own modules and unifying these into a programdirs enum could make sense something akin to ProjectDirs from this stack overflow post.