extend melisma line through notes marked with standalone _ in lyrics

[oddeyed]

Hi Paul, first of all: thank you for the excellent library.

While working on a personal project using the library, I noticed that the lyrics continuation line doesn't appear when it is present in the input w: field.

According to the standard for ABC 2.1, a standalone _ in a w: field should draw a continuation line under the note. For instance, in example 5 in the Lyrics section on this page, the "ble" is continued until the final note in the line.

Instead, in a similar arrangement, the output from abcjs looks like this, where the should be underscore continuations.

This fix doesn't do exactly the same behaviour as in the reference above, but instead by setting both the divider and the syllable in the parser output to _, each continuation syllable has an underscore which sets a clearer intent for the reader of the music - as shown in the new output below.

This PR adds a parse test covering both single and multiple consecutive _ usage.

I'll acknowledge that I wrote this with Claude Code but I couldn't see an explicit AI-assisted code generation policy for your project.

[oddeyed]

I forgot to include the input that generated my above examples:

X: 21
L:1/8
M:none
K:C
C (ED) C (CDEF GAGE DGE D2)
w: mu-NACH_ pa-ZER___________