bug: Adhere to XDG Spec

[ProfessorManhattan]

Checks

• [X] I have searched github.com/withfig/fig/issues and there are no duplicates of my issue
• [X] I have run fig doctor in the affected terminal session
• [X] I have run fig restart and replicated the issue again

Operating system

macOS

Expected behaviour

The ~/.fig folder should be created at ${XDG_CONFIG_HOME:-$HOME/.config}/fig or split up among various XDG locations.

Actual behaviour

Currently, it is creating ~/.fig in the home folder.

Steps to reproduce

No response

Environment

No response