expand repeated lrc timestamps into their own lines

[holofermes]

LRC allows a repeated line to carry several timestamps, it's in the first example of the basic format on https://en.wikipedia.org/wiki/LRC_(file_format), and LRCLIB serves such lines verbatim:

[00:57.32][01:51.00][02:44.20]Oh deserts down below us

ParseLrcLine only reads the first tag, so the other tags end up as lyric text and the later repetitions just don't exist. As of now, I found Beck's Paper Tiger has this in.

This makes ParseLrcLine return one Line per timestamp with the tags stripped from the words, and both providers sort after expanding since the repeats land out of order. That does change the signature from Line to []Line.

Totally fine keeping the old signature instead of breaking the API.