Implement strongly typed cast from/to JSValue

[vmoroz]

This PR implements checked and unchecked casting between JSValue and other types that implement the new IJSValue interface.

• Added new IJSValue interface which is implemented by JSValue and types that wrap up JSValue such as JSArray, JSDate, etc.
• The IJSValue interface new generic methods: Is<T>, As<T>, AsUnchecked<T>, and CastTo<T>.
• These methods are implemented in derived structs with help of IJSValue static methods CanCreateFrom and CreateUnchecked. In case if we targeted .Net version is below version 7.0, it uses IJSValueShim helper class that binds to static methods using reflection.
• All IJSValue derived types implement these casting instance methods and the helper static methods.
• Explicit casting operators are changed to use the new JSValue.As<T> method.
• New unit tests are added to test the new casting operations.

Please note that I could not find a good way to differentiate Proxy from Object and currently any JSValue that is an object can be casted to JSProxy.