Working with `Transition.params` is unwieldy

[csvn]

This issue is just for improving Typescript experience. There is no functional changes intended.

When working with Typescript, using Transition.params() usually feels a bit clunky. A piece of code to illustrate the issue:

let toParams = transition.params();

console.log(toParams.id); // Warning; "prop id does not exist on type.."
console.log(toParams['id']); // ugly workaround

Both of the options above leaves the user with no intellisense in their editor. There is solution to the above, which is to cast params to interface/type, but this is messier (especially in one-liners):

let toParams = transition.params() as MyStateParams;

console.log(toParams.id); // yay!
console.log((transition.params() as MyStateParams).id); // not very pretty...

Ideally, I would like to be able to do this:

let toParams = transition.params<MyStateParams>();

console.log(toParams.id);
console.log(transition.params<MyStateParams>().id); // feels better than casting