Intent to ship: Iterator Sequencing

[Daniel Minor]

As of Firefox 146, I intend to enable the Iterator Sequencing proposal by default on all platforms. It is currently available in Nightly builds, but disabled by default and controlled by the `javascript.options.experimental.iterator_sequencing` preference. The proposal adds a new `Iterator.concat` method that builds an iterator over its iterable arguments. Our implementation was contributed by André Bargull, thank you!

The proposal reached Stage 3 of the TC39 process at the July 2025 plenary, Safari shipped their implementation in 18.4 (https://developer.apple.com/documentation/safari-release-notes/safari-18_4-release-notes), work does not appear to have started in Chromium (https://issues.chromium.org/issues/434977727).

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1986672

Standard: https://github.com/tc39/proposal-iterator-sequencing