A txt to m3u8 conversion is often searched by users that would like to create a multimedia playlist from a text source, perhaps links to radio stations send via e-mail or similar source.
If you know the syntax for a .m3u8 playlist file, you could manually create it from a text source or write it completely down using a text editor. But you probably won't find a convertor that would process any text file (.txt) and generate a .m3u8 playlist.