Use --init-directory in Emacs 29+ (#191)

* Use `--init-directory` in Emacs 29+

This will allow a few advantages:
- We can load `early-init.el` file properly, fixing Flash Of Unstyled
Contents (FOUC) issues and improving the performance slightly
- Eventually we can drop `default.el` file loading in Home-Manager
module. Not done here since this would complicate the code without
necessity, however this can be done once Emacs 29+ is default

* Fix build in Emacs <29

* Add #checks.<arch>.init-example-el-emacsGit

* Add .#checks.x86_64-linux.init-example-el-emacsGit to CI

* Fix review issues

* Move config files to share/emacs.d

* Separate tests in two steps

* Add TODOS

* Simplify emacs-dir generation

modules/home-manager.nix

@@ -91,6 +91,7 @@ in
     in
     mkMerge ([
       {
+        # TODO: remove once Emacs 29+ is released and commonly available
         home.file.".emacs.d/init.el".text = ''
           (load "default.el")
         '';