[Enhancement] Read config file from platform-specific config folder

[h-sigma]

By using dirs::config_dir, we use a more common standard for finding/storing config files, one that doesn't pollute the end-user's home directory. Examples:

Linux: /home/h-sigma/.config/.rustscan.toml # $XDG_CONFIG_HOME or $HOME/.config
MacOS: /Users/h-sigma/Library/Application\ Support/.rustscan.toml # $HOME/Library/Application Support
Windows: C:\Users\h-sigma\AppData\Roaming # {FOLDERID_RoamingAppData}

I have kept the old path in the output messages and the code, for backwards compatibility, and if the config_dir doesn't have the config file, the code checks for the current solution /home/$USER/.rustscan.toml.