Fix "Adding a playlist via 'QueuedLavalinkPlayerExtensions' does not work"

[NycroV]

Fixes #185

Removes the compound null replacement assignment in favor of regular assigning. The default result value will always be null anyways, so we don't need it.

Adds documentation, and specifies that in the event a playlist is passed to this method, the result returned will be the index of the last track added to the queue, not the first. This is more useful for things like determining queue length with less asynchronous executions in certain contexts, and generally is more useful to end users. When shuffling tracks that are added to the queue, this return result is irrelevant anyways.

Ex: if a player is currently playing, but has an empty track queue, and a playlist with 5 tracks is played, track index 5 should be returned.

It may be worth adding support for determining whether you want the first or last index returned in the future, but it could also potentially be seen as clogging the API with unnecessary methods. Will have to see as time goes on.

[github-actions]

Delta Summary

Generated on:	09/23/2024 - 04:02
Description	Previous	Current	Delta
Coverage date:	09/12/2024 - 18:05	09/23/2024 - 04:02
Line coverage:	64.5%	66.4%	1.9%
Covered lines:	17818	14694	-3124
Coverable lines:	27584	22106	-5478
Total lines:	60678	60595	-83
Branch coverage:	50.2%	51.2%	1.0%
Covered branches:	2534	2512	-22
Total branches:	5045	4901	-144
Method coverage:	Feature is only available for sponsors

[NycroV]

I'll probably merge this PR in the next few days if no one has any issues. The code changes are very minor, and it's been verified functional. Small bug fix!