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/ex_cmds.h b/src/ex_cmds.h
index 196cdcf..c124a1e 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1378,6 +1378,9 @@
 EXCMD(CMD_spellgood,	"spellgood",	ex_spell,
 	EX_BANG|EX_RANGE|EX_NEEDARG|EX_EXTRA|EX_TRLBAR,
 	ADDR_OTHER),
+EXCMD(CMD_spellrare,	"spellrare",	ex_spell,
+	EX_BANG|EX_RANGE|EX_NEEDARG|EX_EXTRA|EX_TRLBAR,
+	ADDR_OTHER),
 EXCMD(CMD_spelldump,	"spelldump",	ex_spelldump,
 	EX_BANG|EX_TRLBAR,
 	ADDR_NONE),