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 #4291)
diff --git a/src/normal.c b/src/normal.c
index f717e8c..2067542 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -5127,7 +5127,8 @@
 		    if (ptr == NULL && (len = find_ident_under_cursor(&ptr,
 							    FIND_IDENT)) == 0)
 			return;
-		    spell_add_word(ptr, len, nchar == 'w' || nchar == 'W',
+		    spell_add_word(ptr, len, nchar == 'w' || nchar == 'W'
+					      ? SPELL_ADD_BAD : SPELL_ADD_GOOD,
 					    (nchar == 'G' || nchar == 'W')
 						       ? 0 : (int)cap->count1,
 					    undo);