v0.15.1 — closure data the collector frees

Closure data the collector frees. The fix 0.15.0 made for promise
continuations, applied to plain function closures: JsFunction.create_with_data
gains a finalize_cb overload that ties heap data to the function's lifetime,
and the demo createAdder, which leaked its capture on every call, uses it.

No existing Mojo signature changed — the overload is an addition — so
downstream addons compile against 0.15.1 unmodified. createAdder gains an
optional second argument; calling it with one behaves as before.

Added

• JsFunction.create_with_data(b, env, name, cb_ptr, data, finalize_cb) —
  the closure form for heap data. The collector runs finalize_cb once the
  function is gone; data is adopted on every path, so if the function
  cannot be created or the finalizer attached, finalize_cb runs before the
  call raises. Nothing is freed on the call path, because a function may be
  called many times or never. The existing five-argument form is unchanged
  and still frees nothing. JsPromise.on_settled is now built on this
  overload, so the adopt-on-failure logic and the one function-pointer cast
  it needs live in a single place.

Fixed

• createAdder leaked its capture on every call. The closure example the
  docs pointed to allocated its data per call and handed it to a function
  with no finalizer, so nothing ever freed it — the same defect class 0.15.0
  fixed for promise continuations. It now uses the finalize_cb overload. It
  takes an optional counter ArrayBuffer(8) whose Int64 the capture's
  finalizer increments, and GC tests pin both halves: the capture is freed
  when the adder is collected, and is not freed while the adder is reachable.
  Both regressions were mutation-checked against the suite.