Print the AggregateError header before iterating its members - #35174

Related gap (pre-existing, not a regression): an `AggregateError` reached via `.cause` still omits its member errors. The `.errors` iteration lives only in `print_errorlike_object`, but the `.cause` recursion at ~line 6092 calls `print_error_instance_js` → `print_error_instance_body` directly, and `.errors` is `DontEnum` so the own-property loop skips it — so `throw new Error("outer", { cause: new AggregateError([new Error("inner")], "agg") })` prints `AggregateError: agg` but never `inner`. Wor

The unconditional `writer.write_all(b"\n")` prepend gives one blank line between the header and the first member but *two* blank lines between consecutive `BuildMessage`/`ResolveMessage` members, because those already emit a trailing blank line of their own — the regenerated snapshot in `jsx-template-string-crash.test.ts` captures the double blank. Purely cosmetic (the pre-PR spacing between members was one blank line); could be tightened by tracking whether the previous member already ended in