regex: single-digit backreferences \1..\9 (jolt-xtss)

\1..\9 now match the text captured by the corresponding group, so
patterns like ([-*_])\1\1 or (\w+) \1 work. The parser records which
groups are referenced; a referenced group additionally captures its text
under a tag and the backref compiles to a PEG (backmatch). Only referenced
groups change — they match possessively (the CPS-over-possessive-PEG engine
can't backtrack into a tagged capture), so backtracking back into a
backreferenced group isn't supported (rare). Unreferenced groups keep full
backtracking and position-based result capture unchanged.

test/spec/regex-spec.janet

@@ -72,3 +72,12 @@
   # expand to ~2^61 nodes and hang at compile time.
   ["nested bounds compile" "true"
    "(boolean (re-matches #\"[a-z](?:[a-z]{0,61}[a-z])?(?:-[a-z]{0,61}[a-z])*\" \"abc-def\"))"])
+
+(defspec "regex / backreferences (jolt-xtss)"
+  ["repeated char"      "true"   "(boolean (re-find #\"(.)\\1\" \"abba\"))"]
+  ["no repeat = nil"    "nil"    "(re-find #\"(.)\\1\" \"abc\")"]
+  ["thematic-break run" "true"   "(boolean (re-matches #\"([-*_])\\1\\1\" \"---\"))"]
+  ["mismatched run"     "nil"    "(re-matches #\"([-*_])\\1\\1\" \"-*_\")"]
+  ["repeated word"      "[\"the the\" \"the\"]" "(re-find #\"(\\w+) \\1\" \"say the the word\")"]
+  ["group still captures" "[\"x=x\" \"x\"]" "(re-matches #\"(\\w+)=\\1\" \"x=x\")"]
+  ["html tag pair"      "true"   "(boolean (re-matches #\"<(\\w+)>.*</\\1>\" \"<b>hi</b>\"))"])