vim-patch:8.1.1838: there is :spellwrong and :spellgood but not :spellrare

Problem:    There is :spellwrong and :spellgood but not :spellrare.
Solution:   Add :spellrare. (Martin Tournoij, closes vim/vim#4291)
08cc374dab

src/nvim/ex_cmds.lua

@@ -2549,6 +2549,12 @@ module.cmds = {
     addr_type='ADDR_OTHER',
     func='ex_spell',
   },
+  {
+    command='spellrare',
+    flags=bit.bor(BANG, RANGE, NEEDARG, EXTRA, TRLBAR),
+    addr_type='ADDR_OTHER',
+    func='ex_spell',
+  },
   {
     command='spelldump',
     flags=bit.bor(BANG, TRLBAR),