[REQUEST] Respect XDG Base Dir Specification Environment Variables

[Neurognostic]

Is your feature request related to a problem? Please describe.
bpytop currently does not respect the setting of the XDG Base Directory Specification environment variables.

Additionally, bpytop does not even provide its own environment variable or command line flag to change the configuration directory. So no workaround through aliases or other environment variables is possible without editing the source code.

Describe the solution you'd like
Specifically, the configuration directory should check for the XDG_CONFIG_HOME environment variable and use it if it exists and is set to a valid directory path, otherwise use a default.

Describe alternatives you've considered
Other applications that do not support the XDG Base Directory Specification almost always have some method to force it to respect the XDG spec; for example, using aliases with a command line argument that points to a different configuration directory or a configuration directory environment variable that users can assign to the value of $XDG_CONFIG_HOME.

Additional context
There are other environment variables in the XDG Base Directory Specification that bpytop should try to respect when reading or writing files other than config files, such as when writing log files.