Feature request: allow squiggle lines for line spanners

Requested by a French-speaking user here: https://lists.gnu.org/archive/html/lilypond-user-fr/2021-10/msg00027.html

The currently supported styles for line spanners are line, dashed-line, dotted-line, zigzag and trill. It would be nice to support squiggle-line as well, reusing the \draw-squiggle-line markup commands.

https://lists.gnu.org/archive/html/lilypond-user-fr/2021-10/msg00028.html suggests a workaround working in the case of a text spanner without arrows.