Use XDG directory on Linux.

[alula]

Description

Fixes #558.

I personally think that polluting the home directory with dotfiles is a pretty lazy and bad practice.

This PR currently only addresses said problem on Linux, by making Continue follow the XDG Base Directory specification.

This change is backwards compatible and only affects new setups, ~/.continue is still used if it already exists.

I didn't do anything about other platforms yet, Windows and macOS continues to use ~/.continue just like before.

Checklist

• [x] The base branch of this PR is dev, rather than main
• [x] The relevant docs, if any, have been updated or created