BigInt.prototype.toLocaleString() with no arguments skips default-locale grouping

[proggeramlug]

BigInt.prototype.toLocaleString() called with no arguments does not apply the default locale's digit grouping.

const b = 12345678901234567890n;
console.log(b.toLocaleString());        // perry: 12345678901234567890
                                        // node:  12,345,678,901,234,567,890
console.log(b.toLocaleString('de-DE')); // both:  12.345.678.901.234.567.890
console.log((9876543n).toLocaleString('en-US')); // both: 9,876,543

Passing an explicit locale works correctly, so the formatter itself is fine — the no-argument path is not resolving to the default locale before formatting. Number.prototype.toLocaleString is unaffected ((1234567.891).toLocaleString() prints 1,234,567.891, matching Node), because it routes through the hand-rolled grouper in date.rs rather than intl::bigint_to_locale_string.

Reproduced on a pristine build of bd7cbb7da in a separate worktree, so this predates the current main.

Found while gating the thunk for binary size in #7427; that PR does not change this path (the ECMA-402 branch there is byte-for-byte unchanged).

[proggeramlug]

Verified fixed independently on current main (1ee158d). The exact parity probe includes separate zeroarg: and undef: BigInt rows plus a smaller zero-argument value; it passes byte-for-byte against Node (1/1 pass, 0 failures) with the current 0.5.1463 compiler/runtime archives. Merge commit bec70d6 from #7797 is present on main. Closing as fixed by #7797.