mprisplugin: xesam:artist is a list of strings

See also
- https://searchfox.org/mozilla-central/rev/a3c18883ef9875ba4bb0cc2e7d6ba5a198aaf9bd/dom/media/mediasession/MediaMetadata.h#34
- https://source.chromium.org/chromium/chromium/src/+/main:services/media_session/public/cpp/media_metadata.h;l=46;drc=098756533733ea50b2dcb1c40d9a9e18d49febbe