Allow an interface in the JSX namespace to specify the element class type

We're writing a non-react library that uses TS's JSX support with custom interfaces defined in the JSX namespace. Unless we are very much mistaken (The documentation seems a little out-of-date in this area? Most of our current understanding comes from reading through issues on here, we may have missed something.) it seems that while ElementClass can be used to define the type of the element instance type there is no interface to limit the members of the element class type.

We have a custom component base-class and an equivalent to React's createElement function. Our createElement function needs to be able to create instances of the component classes. To be able to do this in a type-safe manner, we need to be able to specify an interface for the element class type that can require a specific constructor signature.

This feels like an oversight in the current implementation TBH. With React itself it's even possible to create components with incompatible constructors that would fail at runtime.

Start by reviewing TypeScript's JSX namespace support, especially ElementClass and the custom createElement scenario described in the issue. Determine how an interface for the element class type should express a constructor requirement. Done means JSX type checking can enforce that requirement for custom components without breaking existing JSX behavior.