Add gjs_context_run_in_realm() API

Merged Philip Chimento requested to merge ptomato/gjs:run-in-realm into master

This merge request consists of a new C API (and a bug fix to make it possible) which allows using SpiderMonkey API to do things in the GJS realm, for advanced use cases. Most C programs that embed GJS should not use the SpiderMonkey API directly, because it's not API stable. But in some cases, for example running some JS code and manipulating the JS return value, it's better than the alternatives.

Closes: #536 (closed)

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