[@types/ssh2-streams] Fix parseKey return type (#44261)
[@types/ssh2-streams] Fix parseKey return type (#44261)

`parseKey` can return a key ([via](https://github.com/mscdex/ssh2-streams/blob/4bc26e937551fb86f03b24c397f738ac2a9dcae8/lib/keyParser.js#L1444) [`PPK_Private.parse()`](https://github.com/mscdex/ssh2-streams/blob/4bc26e937551fb86f03b24c397f738ac2a9dcae8/lib/keyParser.js#L1209)), or `null` (see same function), or `Error` (trivially in `parseKey`), or an array of keys (via [`OpenSSH_Private `](https://github.com/mscdex/ssh2-streams/blob/4bc26e937551fb86f03b24c397f738ac2a9dcae8/lib/keyParser.js#L639)
dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped