Add ability to load lyrics from .txt files in addition to .lrc files

Reason for the change

Regular unsynchronized song lyrics may be stored in a regular plain text .txt file instead of an LRC lyric file. This merge request extends functionality implemented in commit f8602efd enabling use of .txt files for song lyrics.

Test plan

Manual testing should be sufficient as this request only changes what files are picked up by Elisa as lyric files:

1. Add a .txt file to a directory containing an audio file with the same name except for the file extension.
2. Open Elisa and play said audio file.
3. Observe the contents of the text file showing up as song lyrics.