emacs: devel: Add indentation rules for 'modify-phases' keywords.

* emacs/guix-devel.el: Add indentation rules for 'modify-phases' keywords.
  (guix-devel-indent-modify-phases-keyword,
  guix-devel-indent-modify-phases-keyword-1,
  guix-devel-indent-modify-phases-keyword-2): New functions.