ujs: migrate to TypeScript

[liamwhite]

This moves all of the ujs events convenience library into TypeScript. There should be no substantial changes to the logic here, but two bugs were discovered during testing and fixed:

1. Incorrect use of target where input was intended.

   $$('[disabled][data-disable-with][data-enable-with]', target).forEach(input => {
     // ...
     input.innerHTML = target.dataset.enableWith;
     //                ^^^^^^ input
   }

2. HTMLFormElement's method property is restricted, so the final 'POST' case is unreachable here:

    method: (target.dataset.method || target.method || 'POST').toUpperCase(),

This has 100% test coverage in unit tests and has also been manually integration tested.

[SeinopSys]

You could use mockResolvedValue with the newly created Response instance to avoid the use of any

[liamwhite]

I ended up changing this to mockReturnValue. I need to defeat the type system so that there is no async code, because the function I am testing cannot return anything.