Add a way to get constructor of any imported type as a value

[RReverser]

Motivation

From #1403 which added Object::constructor property:

but the problem is that you can't do comparisons like any_value.constructor() == Array::constructor_as_a_function_or_something_like_that() because there is no method to get a constructor of a statically defined type.

These comparisons could be useful when one wants to check if particular value has particular type's constructor.

Proposed Solution

Add something like wasm_bindgen::get_constructor<T>() -> JsValue that could be used on any imported type to get it as an actual value (the constructor itself).

Alternatives

Statically generate some method on each imported type?