[SM68 2/2] Update to Spidermonkey 68 APIs

No noteworthy API changes.

Details:

• Remove UTF16 script execution since UTF8 is supported in SM68 and going forward
• Several new headers includes are required
• Realms replace Compartments as "global holders" (see meta-Bug 1357862)
• JSRequests are removed entirely (Bug 722345), see also rP24180
• Trivial API updates in ProxyHandlers, ArrayBuffer, Warnings, GC reasons, Context options, ObjectIsFunction, ValueVectors and JSCompartment

See also the migration guide: https://github.com/mozilla-spidermonkey/spidermonkey-embedding-examples/blob/esr78/docs/Migration%20Guide.md

Tested by: Freagarach, Stan, Subitaneo

Fixes #5860

Differential Revision: https://code.wildfiregames.com/D3144