Dom Ready: Add types

[sirreal]

Description

Part of #18838
Extracted from #18942

Add types to dom-ready package.

Testing Instructions:

Verify type checking passes:

npm run lint-types

[sirreal]

Subtle behavioral change - this function did not always return void as its type suggested. In this branch, tt would return whatever the callback returned.

Adding the void here so the function always returns undefined.

[aduth]

Subtle behavioral change - this function did not always return void as its type suggested. In this branch, tt would return whatever the callback returned.

Adding the void here so the function always returns undefined.

I think this syntax can be a little unexpected for some people, or at least it was for me the first times I had seen it, but I think it's a nice simple way to guarantee the return type as undefined, as you suggest.

[aduth]

This will need a rebase after #19099 was merged (it will take some time for the "include" to populate a bit more before we can avoid such frequent merge conflicts 😬 ).

[sirreal]

Rebased.