Properly encode non-lyric text

Currently text included in the source that isn't actually part of the lyrics is encoded as a syllable element only containing a `syl` element and no neumes. If a block of text of this kind occurs, it should instead be encoded as text separately from the musical elements. How to do this is described in [Chapter 9 (Text Encoding) of the MEI Guidelines](https://music-encoding.org/guidelines/v4/content/textencoding.html).