Add config_dir-based file option

[CosmicHorrorDev]

Resolves #394

This uses the dirs crate to add <config_dir>/kanata/kanata.kbd to the config path

default_cfg() checks to see if the file exists before adding it to the path because the check later in cli_init() specifically checks the first config file. There is also a new error message for providing no config files provided since an empty vec of config files can be passed through now

[rszyma]

Configuration file(s) to use with kanata. If not specified, defaults to
kanata.kbd in the current working directory and <config_dir>/kanata/kanata.kbd.

It might be a good idea to show actual path of the config dir rather than literally saying "<config_dir>" as it might be confusing to user where the <config_dir> is located.

[CosmicHorrorDev]

That gets a bit messy with how clap's derive API is configured unfortunately. I'll look at how other programs document this to see if that gives me any better ideas

[jtroo]

Perhaps documenting it as ... $XDG_CONFIG_HOME/kanata/kanata.kbd. "$XDG_CONFIG_HOME" is usually "$HOME/.config". might be good enough?

[CosmicHorrorDev]

That seems like a good middle ground to me

[rszyma]

Isn't ~/.config/kanata/kanata.kbd always the same path as $XDG_CONFIG_HOME/kanata/kanata.kbd?

Another thing, dirs::config_dir also works on windows, so instead of this:

    /// kanata.kbd in the current working directory and $XDG_CONFIG_HOME/kanata/kanata.kbd.
    #[arg(short, long, verbatim_doc_comment)]
    cfg: Option<Vec<PathBuf>>,

We could do this:

    #[cfg(target_os = "windows")]
    /// Configuration file(s) to use with kanata. If not specified, defaults to
    /// kanata.kbd in the current working directory and C:\Users\<USER>\AppData\Roaming\kanata\kanata.kbd.
    #[arg(short, long, verbatim_doc_comment)]
    cfg: Option<Vec<PathBuf>>,

    #[cfg(not(target_os = "windows"))]
    /// Configuration file(s) to use with kanata. If not specified, defaults to
    /// kanata.kbd in the current working directory and ~/.config/kanata/kanata.kbd. 
    #[arg(short, long, verbatim_doc_comment)]
    cfg: Option<Vec<PathBuf>>,

[CosmicHorrorDev]

Well it wouldn't be the same path if someone sets a different value for $XDG_CONFIG_HOME of course, but by default yes

You could also get cleaner cfgs, by doing something like

#[cfg_attr(target_os = "windows", doc = "I'm on windows")]
#[cfg_attr(not(target_os = "windows"), doc = "I'm not on windows")]

To avoid duplicating the struct's field

And then we'll also have to add one for Mac because they also use a different path too. I guess the paths are pretty stable, but not sure if it's worth the near copy-paste docs

[jtroo]

Fair points on the cross platform issues. Another idea is to revert to the original and link to https://docs.rs/dirs/latest/dirs/fn.config_dir.html for the reference of <config_dir>. Or just add all 3 OS' typical directories in one documentation item.

[CosmicHorrorDev]

I opted for just displaying the platform specific paths of the big three (and assumed that other platforms would likely follow Linux's style). dirs's docs showed {FOLDERID_RoamingAppData} as the value for Windows, but I'm not familiar enough to know if that's an actual path or not, so I opted for just displaying an example path

I think this is a good balance of being direct while still showing the value for platforms other than Linux