Letter tablature formatting

Tablature can be formatted using letters instead of numbers.

\version "2.24.0"

music = \relative c {
  c4 d e f
  g4 a b c
  d4 e f g
}

<<
  \new Staff {
    \clef "G_8"
    \music
  }
  \new TabStaff \with {
    tablatureFormat = #fret-letter-tablature-format
  }
  {
    \music
  }
>>