Vertically centering paired figured bass extenders

Where figured bass extender lines are being used by setting useBassFigureExtenders to #t, pairs of congruent figured bass extender lines are vertically centered if figuredBassCenterContinuations is set to #t.

\version "2.24"

<<
  \relative c' {
    \repeat unfold 3 {
      c8 c b b a a c16 c b b
    }
  }
  \figures {
    \set useBassFigureExtenders = ##t
    <6+ 4 3>4 <6 4 3>8 r
    <6+ 4 3>4 <6 4 3>8 <4 3+>16 r
    \set figuredBassCenterContinuations = ##t
    <6+ 4 3>4 <6 4 3>8 r
    <6+ 4 3>4 <6 4 3>8 <4 3+>16 r
    \set figuredBassCenterContinuations = ##f
    <6+ 4 3>4 <6 4 3>8 r
    <6+ 4 3>4 <6 4 3>8 <4 3+>16 r
  } 
>>