Intent to ship: Error.isError

[Daniel Minor]

As of Firefox 138, I intend to enable Error.isError by default on all platforms. It is currently available in Nightly builds, disabled by default behind the `javascript.options.experimental.error_iserror` preference. The Error.isError proposal allows determining whether the argument provided is in fact an instance of an error (either a JS Error object or a DOMException). Our implementation was contributed by a volunteer, Serah Nderi. Thanks Serah :)

The proposal is currently at Stage 3 of the TC39 process. Chromium support is planned for 134 (https://issues.chromium.org/issues/382104870), Safari shipped in 18.4 (https://developer.apple.com/documentation/safari-release-notes/safari-18_4-release-notes).

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1952249

Standard: https://github.com/tc39/proposal-is-error