Add new keywords SNAPSHOT and TYPES to the keyword list in gram.y

These were added to kwlist.h as unreserved keywords in separate patches,
but authors forgot to add them to the corresponding list in gram.y.
Because of that, even though they were supposed to be unreserved keywords,
they could not be used as identifiers. src/tools/check_keywords.pl is your
friend.

Author: hlinnaka

src/backend/parser/gram.y

@@ -12302,6 +12302,7 @@ unreserved_keyword:
 			| SHARE
 			| SHOW
 			| SIMPLE
+			| SNAPSHOT
 			| STABLE
 			| STANDALONE_P
 			| START
@@ -12325,6 +12326,7 @@ unreserved_keyword:
 			| TRUNCATE
 			| TRUSTED
 			| TYPE_P
+			| TYPES_P
 			| UNBOUNDED
 			| UNCOMMITTED
 			| UNENCRYPTED