Remove JS_New in favour of JS::Construct in preparation for SM91

Spidermonkey 91, the next ESR, removes JS_New in favour or JS::Construct.
See
https://github.com/mozilla-spidermonkey/spidermonkey-embedding-examples/blob/migration-guide/docs/Migration%20Guide.md#object-construction
and
https://bugzilla.mozilla.org/show_bug.cgi?id=1491055

This change is SM78 compatible and therefore done beforehand.

Tested by: Freagarach

Refs #5986

Differential Revision: https://code.wildfiregames.com/D4427