feat: parallelism and fix: 404 unauthorized

[eudesmedeiros7]

I'd like to make a suggestion to developers: add parallelism to the code. Perhaps developing this feature could help speed up transfers. I'm running the program right now and it's taken me 38 minutes to download liked songs and over 40 minutes for saved playlists.

The program also had a problem with 404 authorization which I solved by searching the issues here on github (I used the ytmusicapi browser command to generate a json browser and then modified it in the lib's backend.py file). Here's a suggestion for fixing it too.