Respect XDG environment variables for .aftman folder

[4xMSAA]

Rather than placing the .aftman folder directly into HOME, I'd much prefer if it tried out the XDG_DATA_HOME environment variable first, then fall back to HOME.

It may also be a little better to separate aftman.toml into XDG_CONFIG_HOME which would make backups easier.

Additionally, perhaps make use of an environment variable named AFTMAN_HOME, much like RUSTUP_HOME and CARGO_HOME are (although this may pollute the env, it is still better than not having an option where .aftman is found at all)