patch 9.1.1892: Not possible to know once Vim is done with sourcing vimrc

patch 9.1.1892: Not possible to know once Vim is done with sourcing vimrc

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: #18668

Signed-off-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>