String number extender lines

Make an extender line for string number indications, showing that a series of notes is supposed to be played all on the same string.

\version "2.24"

stringNumberSpanner =
  #(define-music-function (StringNumber) (string?)
    #{
      \override TextSpanner.style = #'solid
      \override TextSpanner.font-size = #-5
      \override TextSpanner.bound-details.left.stencil-align-dir-y = #CENTER
      \override TextSpanner.bound-details.left.text =
        \markup { \circle \number $StringNumber }
    #})

\relative c {
  \clef "treble_8"
  \textSpannerDown
  \stringNumberSpanner "5" a8\startTextSpan b c d
    e f\stopTextSpan \stringNumberSpanner "4" g\startTextSpan a |
  bes4 a g2\stopTextSpan
}