vim-patch:9.1.1892: Not possible to know once Vim is done with sourcing vimrc (#36429)

Problem:   A plugin does not know when startup scripts were already
           triggered. This is useful to determine if a function is
           called inside vimrc or after (like when sourcing 'plugin/'
           files).
Solution:  Add the v:vim_did_init variable (Evgeni Chasnovski)

closes: vim/vim#18668

294bce21ee

Nvim has two more steps between sourcing startup scripts and loading
plugins. Set this variable after these two steps.

Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>

src/nvim/vvars.lua

@@ -901,6 +901,14 @@ M.vars = {
       Read-only.
     ]=],
   },
+  vim_did_init = {
+    type = 'integer',
+    desc = [=[
+      0 during initialization, 1 after sourcing |vimrc| and just
+      before |load-plugins|.
+      Read-only.
+    ]=],
+  },
   virtnum = {
     type = 'integer',
     desc = [=[