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/eval/vars.c

@@ -203,6 +203,7 @@ static struct vimvar {
   VV(VV_EXITING,          "exiting",          VAR_NUMBER, VV_RO),
   VV(VV_MAXCOL,           "maxcol",           VAR_NUMBER, VV_RO),
   VV(VV_STACKTRACE,       "stacktrace",       VAR_LIST, VV_RO),
+  VV(VV_VIM_DID_INIT,     "vim_did_init",     VAR_NUMBER, VV_RO),
   // Neovim
   VV(VV_STDERR,           "stderr",           VAR_NUMBER, VV_RO),
   VV(VV_MSGPACK_TYPES,    "msgpack_types",    VAR_DICT, VV_RO),

src/nvim/eval_defs.h

@@ -123,6 +123,7 @@ typedef enum {
   VV_EXITING,
   VV_MAXCOL,
   VV_STACKTRACE,
+  VV_VIM_DID_INIT,
   // Nvim
   VV_STDERR,
   VV_MSGPACK_TYPES,

src/nvim/main.c

@@ -466,6 +466,8 @@ int main(int argc, char **argv)
     syn_maybe_enable();
   }
 
+  set_vim_var_nr(VV_VIM_DID_INIT, 1);
+
   // Read all the plugin files.
   load_plugins();
 

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 = [=[