Typing for arrayFormatSeparator is wrong in StringifyOptions #247
Description
On this line: https://github.com/sindresorhus/query-string/blob/master/index.d.ts#L64 the type is set to be 'string' but it should be a string. This typing prevents setting any separator other than the literal string "string" which I think was an easy mistake to make but a mistake none-the-less :)