Make `jsi::Object` constructor explicit (#53521)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53521

Make `jsi::Object` constructor explicit, so its creation is explicit and
intentional. This prevents any sad foot-gun of constructing an Object
implicitly from a Runtime, which is certainly not a JS object.

Changelog: [Internal]

Reviewed By: avp

Differential Revision: D81274439

fbshipit-source-id: 5a9d9907f9deff7625dcff9c1072eb135ab7840e