Add `$XSHE_FILE` environment variable to customize the default location of `xshe.toml`

[superatomic]

The environment variable $XSHE_FILE should be able to set where xshe checks for its configuration file if none of the flags -p, -t, or -f are set.

Right now, the program checks locations in this order:

• $XDG_CONFIG_HOME/xshe.toml, if $XDG_CONFIG_HOME is set.
• ~/.config/xshe.toml.

The desired behavior is add the following check before the other 2:

• $XSHE_FILE, if $XSHE_FILE is set.