Incorrect "setSelectionRange()" type declarations

[mariusschulz]

lib.dom.d.ts currently has two different type declarations for the setSelectionRange() API for HTMLInputElement and HTMLTextAreaElement.

For HTMLInputElement (permalink):

setSelectionRange(start?: number, end?: number, direction?: string): void;

For HTMLTextAreaElement (permalink):

setSelectionRange(start: number, end: number): void;

However, both are wrong, if I'm not mistaken. According to MDN, the correct type declaration would be the following:

setSelectionRange(start: number, end: number, direction?: "forward" | "backward" | "none"): void;

Note that neither start nor end are optional. I also added a union of string literal types to type the (optional) direction parameter more precisely.

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[mariusschulz]

Alright, here's the PR: #20459