Add rules for nth and only-tags. Address #1503

These rules allow selecting from a number of sub-captures
while dropping the rest. `nth` is more succinct in many cases, but `only-tags` is
more general and corresponds to an internal mechanism already present.

src/include/janet.h

@@ -2180,7 +2180,9 @@ typedef enum {
     RULE_UNREF,        /* [rule, tag] */
     RULE_CAPTURE_NUM,  /* [rule, tag] */
     RULE_SUB,          /* [rule, rule] */
-    RULE_SPLIT         /* [rule, rule] */
+    RULE_SPLIT,        /* [rule, rule] */
+    RULE_NTH,          /* [nth, rule, tag] */
+    RULE_ONLY_TAGS,    /* [rule] */
 } JanetPegOpcod;
 
 typedef struct {