[Feature Request] Alphabetically sort playlist names when using the playlists command

**Is your feature request related to a problem? Please describe.**
Alphabetically sort playlist names when using the `playlists` command.

**Describe the solution you'd like**
Sort the list before sending the message.

**Describe the basic flow/steps of using this feature**
Described above.

**Additional context**
None.

**Have you confirmed that this idea is not already described here?**
https://github.com/jagrosh/MusicBot/projects/1
Yes.

Start at the implementation of the `playlists` command and inspect how playlist names are collected and sent in the message. Sort the list before sending it, then verify that the command displays playlist names alphabetically; check the linked pull request for existing work before starting.

Written by the indexing model from the issue text.