Bug 1719747 - Part 3: ListFormat::FormatToParts takes a callback. r=gregtatum,tcampbell

As discussed in https://bugzilla.mozilla.org/show_bug.cgi?id=1719747#c6,
In Part 1, the ownership of the Span in PartsVector is maintained by
ICU. This method adds a callback so js::intl::ListFormat could copy the
content of the Span to Spidermonkey, and mozilla::intl::ListFormat could
use ScopedICUObject to release the string owned by ICU earlier.

Differential Revision: https://phabricator.services.mozilla.com/D123166