feat: persistent singly-linked lists with O(1) conj/cons prepend

Round 3 of the persistent-collections work. Lists were immutable Janet arrays,
so conj/cons-prepend was an O(n) copy (O(n^2) to build a list) — a large perf
gap vs Clojure's PersistentList.

Add src/jolt/plist.janet: an immutable cons-cell list (first/rest/count), same
algorithm as Clojure/CLJS/jank PersistentList. conj/cons onto a list now creates
an O(1) node that shares the existing list as its tail (no copy), with a cached
O(1) count. Repeated conj is O(n) total instead of O(n^2).

Hooked plist through first/rest/next/seq/count/peek/pop/nth/empty/empty?, the
predicates (list?/seq?/coll?/sequential?), realize-for-iteration, =, coll->cells
(concat/lazy), both printers, destructuring, and instance? tags. (list ...) and
quoted lists stay arrays; only conj/cons introduce plist nodes, so the surface
and risk stay small.

Verified: reduce-conj of 200k elements runs in ~0.4s (was effectively O(n^2)).
conformance 206/206, features 78/78 (+7 list regressions), jank 120 (+1).

test/jank-conformance.janet

@@ -12,7 +12,7 @@
 
 # Baseline: the number of pass-tests Jolt currently handles. Raise this as Jolt
 # gains features so regressions (a previously-passing test breaking) are caught.
-(def baseline 119)
+(def baseline 120)
 
 # Tests that loop forever under Jolt's eager evaluation (skipped to avoid hangs;
 # tracked as known gaps — variadic-recur arity selection and var-quote calls).