Support subclassing Observable with non-class constructor functions.

[benjamn]

Now that the zen-observable-ts package has the ability to export Observable as a native ECMAScript class (#7615), we need to be careful when extending Observable using classes (like ObservableQuery and Concast) that have been compiled to ES5 constructor functions (rather than native classes), because the generated _super.call(this, subscriber) code throws when _super is a native class constructor (#7635).

Rather than attempting to change the way the TypeScript compiler transforms super(subscriber) calls, this commit wraps Observable.call and Observable.apply to work as expected, using Reflect.construct to invoke the superclass constructor correctly, when the Reflect API is available.

Another option would be to ship native class and super syntax with @apollo/client, by changing the "target" in tsconfig.json from "es5" to "es2015" or later, so consumers of @apollo/client code would be forced to compile native
class syntax however they see fit. That would be a more disruptive change, in part because it would prevent subclassing Apollo Client-exported classes using anything other than native class syntax and/or the Reflect.construct API, which is the very same problem this commit is trying to fix for the Observable class.

[benjamn]

Another option: I could move this logic into zen-observable-ts/module.js, so it would only be applied when the native Observable class is imported.