fix: preserve a trailing hole in sparse arrays

[spokodev]

Problem

A sparse array whose last element is a hole loses that hole on a round-trip, so the array comes back shorter:

import { stringify } from "javascript-stringify";

const value = [1, , ]; // length 2 (a hole at index 1)
stringify(value);       // "[1,]"
eval("(" + stringify(value) + ")").length; // 1  ← should be 2

Leading and interior holes are fine ([,1], [1,,3]); only a trailing hole is dropped.

Root cause

arrayToString maps the values (Array.prototype.map preserves holes) and joins them:

const values = array.map(...).join(",");
return `[${eol}${values}${eol}]`;

join renders a hole as an empty string between separators, which works for leading and interior holes. But a trailing hole produces a single trailing comma — and [1,] is a length-1 array literal, so the hole is lost.

Fix

When the last element is a hole, append one more separator so the array literal keeps its length ([1,,] has length 2):

const trailingHole =
  array.length > 0 && !(array.length - 1 in array) ? "," : "";
return `[${eol}${values}${trailingHole}${eol}]`;

Test plan

• Added round-trip tests for trailing, leading and interior holes (the two trailing-hole cases fail on main, the others already passed).
• Full suite green (150).