Add support for XDG base directory for doomemacs and emacs

[psujit775]

All submissions

• I have followed the Contributing Guidelines
• I have checked to ensure there aren't other open Pull Requests for the same update/change

Adding/updating Application X Support

• This PR is only for one application
• It has been added to the list of supported applications in the README
• Changes have been added to the WIP section of the CHANGELOG
• Syncing does not break the application
• Syncing does not compete with any syncing functionality internal to the application
• The configuration syncs the minimal set of data
• No file specific to the local workstation is synced
• No sensitive data is synced

Improving the Mackup codebase

• My submission passes the tests
• I have linted the code locally prior to submission
• I have written new tests as applicable
• I have added an explanation of what the changes do

Ref: Emacs NEWS for 27.1

• Startup Changes in Emacs 27.1

Emacs can now use the XDG convention for init files. The 'XDG_CONFIG_HOME' environment variable (which defaults to
"~/.config") specifies the XDG configuration parent directory. Emacs
checks for "init.el" and other configuration files inside the "emacs"
subdirectory of 'XDG_CONFIG_HOME', i.e. "$XDG_CONFIG_HOME/emacs/init.el"