Vertically centering paired figured bass extenders

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

\version "2.24.0"

<<
  \relative c' {
    c8 c b b a a c16 c b b  
    c8 c b b a a c16 c b b  
    c8 c b b a a c 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+>8
  } 
>>