Add change song command to API

[Setadokalo]

This just adds a command to the API to change the current song/playlist, using logic copied from the existing implementation for the URI handler. There's still useful functionality that won't be covered, such as adding a song to the queue without replacing the queue entirely, but this at least covers a major use case.

There's no validation to ensure the video and playlist id are correct, which can cause some weirdness; invalid playlist ids will cause the bottom bar to disappear, for example.

[NovusTheory]

Just requesting that the command name be changed and it should be good.

Also please make sure to add the error code to the errorCodes array that was above otherwise the API will not properly return it https://github.com/ytmdesktop/ytmdesktop/pull/1355/files#diff-84b5d03309d58f0086ac6d2cb4cdce5f610a13d9f697edfb31974dd3a8c2c9b2R4-R17

[Setadokalo]

Done, with the slight exception of changing song to id; given that either the playlist id or video id can be provided, it feels weird to me to give video id the 'default' name, so I changed it to videoId and playlistId. It looks like that's consistent with internal stuff as well.

[NovusTheory]

LGTM

[Setadokalo]

Squashed the changes into a single commit to make it ready to merge.