Inconsistent signatures for alert() vs prompt()/confirm()

[Gaelan]

Currently, we have:

declare function alert(message?: any): void;
declare function prompt(message?: string, _default?: string): string | null;
declare function confirm(message?: string): boolean;

Presumably alert is typed as taking any because it accepts input of any type and implicitly casts it to a string. However, testing in Chrome, Safari, and Firefox, prompt and confirm both do the same thing.

[saschanaz]

I guess it can be string too, yeah. PRs welcome.