Refactor songtag LRC handling

[hasezoey]

This PR refactors many parts of the lib::songtag::lrc module and touches upon some related lyric things, in more details:

• add tests for LRC parsing and writing
• fix writing milliseconds to LRC format (previously the completely wrong value was used)
• refactor a bunch of stuff to require less heap (string) allocations
• refactor a bunch of stuff to do less (repeated) searching
• general addition of doc-comments
• cleanup unused code / variables / properties

[hasezoey]

I did some more changes:

• add tests for merge_adjacent, adjust_offset and get_text
• change Lyric::get_index to not use a artificial 2 second offset anymore
• fix Lyric::adjust_offset inverting the given offset when below 10 seconds (ie Lyric::adjust_offset(_, 1000) would give self.offset = -1000 instead of self.offset = 1000), this didnt seem intentional, but if it is we should add a comment
• change from casting some values to using try_from
• use milliseconds directly instead of as_secs
• some style changes
• less cloning again
• change tui no lyrics available message to be the else case instead of basing it on another value
• add a note why Lyric::get_text uses a artificial 2 second offset