Change configuration directory

[FlafyDev]

I'd like to have all my helix configuration files not in ~/.config/helix, but in some other directory of my choosing. This is only partly possible with the -c/--config flag that can override the default ~/.config/helix/config.toml path to any other file. But other configuration files in ~/.config/helix (such as languages.toml) cannot be changed and are not affected by the -c/--config flag.

One solution could be to add a new flag that can override the configuration directory.

[the-mikedavis]

We're trying to use as few CLI flags as possible since the program is interactive. (Though -c exists since you need config before you can start the editor.) #2135 allows this without introducing a new CLI flag: you can configure the locations of the other config files/dirs in the config.toml