feat: add .doommodule files

These optional dotfiles indicate the root of a module or module
group (:lang), and will later contain module metadata. They will also
serve as an alternative to packages.el and doctor.el, and will aide the
parts of the v3.0 module API concerned with resolving the current module
from a path (`doom-module-from-path`), which currently rely too heavily
on parsing path strings.

For now, however, they're simply placeholders.

lisp/.doommodule

@@ -0,0 +1,10 @@
+(3 0 0)
+((name . :doom)
+ (depth . -110)
+ ;; (packages
+ ;;  (elpaca :host github
+ ;;          :repo "progfolio/elpaca"
+ ;;          :pin "213a72646e024af007c610ea7574dc82e992637f")
+ ;;  (compat :host github :repo "emacs-compat/compat"
+ ;;          :pin "e9203e164903a6bb7de3e58aa0d653bbcff9d3d1"))
+ )