rename duration to align with ytdl-core

[TimeForANinja]

for ytdl it's named lengthSeconds https://github.com/fent/node-ytdl-core/blob/master/typings/index.d.ts#L126 for ytsr it's named duration and durationSec https://github.com/TimeForANinja/node-ytpl/blob/master/typings/index.d.ts#L42-L43 so it would make sense to rename it to length and lengthSeconds