Editorial annotation improvements (LilyPond Changes)

### Editorial annotation improvements

- Incipits may now be printed with various context types. It also got an optional argument to shift the incipit to the left.

  >     \score {
  >       <<
  >         \new Staff \with { instrumentName = "MensuralStaff" }
  >           {
  >             \incipit { c'4 d' }
  >             c'4 d' e' f' g'1
  >           }

  >         \new Staff \with { instrumentName = "KievanStaff" }
  >           {
  >             \incipit #2 \new KievanStaff { c'4 d' }
  >             c'4 d' e' f' g'1
  >           }
  >         \new Staff
  >           \with { instrumentName = "TabStaff" }
  >           {
  >             \incipit
  >               \new TabStaff
  >                 \with { \magnifyStaff 0.5
  >                         \override InstrumentName.font-size = 6 }
  >                 { c'4 d' }
  >             c'4 d' e' f' g'1
  >           }
  >       >>
  >       \layout {
  >         indent = 5\cm
  >         incipit-width = 2\cm
  >       }
  >     }

- Optional additional material can be enclosed in brackets that pass through the staff with `\startOptionalMaterial` and `\stopOptionalMaterial`.

- The `FingerGlideSpanner` may now connect `StringNumber` and `StrokeFinger` grobs.

  >     {
  >       \set strokeFingerOrientations = #'(down)
  >       b2 \glide -\rightHandFinger #1 \glide -2 \glide \5
  >       e'-\rightHandFinger #1 -2 \5
  >     }

- A new engraver, `Toe_heel_engraver` (enabled by default) listens to the context variable `toeHeelStyle` to provide styling options for the commands `\rtoe`, `\ltoe`, `\rheel`, and `\lheel`.

  >     music = { f'4\rtoe f'4\rheel f'4\ltoe f'4\lheel \bar "||" }

  >     {
  >       \music            % #'default
  >       \set toeHeelStyle = #'standard    \music
  >       \set toeHeelStyle = #'reversed    \music
  >       \set toeHeelStyle = #'below       \music
  >       \set toeHeelStyle = #'circleheels \music
  >     }

  If the engraver is enabled, these commands no longer behave like normal `Script` grobs since they ignore direction changes (i.e., ‘`^`’, ‘`_`’, and `\tweak direction` are ignored). If the need arises to position pedal marks individually, the new, standard articulation commands `\toe`, `\vartoe`, `\heel`, `\varheel`, and `\heelcircle` (accessing the new glyph `scripts.pedalheelcircle`) should be used instead.

  >     {
  >       f'2\toe f'2\vartoe |
  >       f'2\heel f'2\varheel |
  >       f'1\heelcircle
  >     }

  Removing the engraver makes `\rtoe` and siblings behave as in previous LilyPond versions.

- Directly related to the previous item are four new, articulation-like commands to indicate toe-heel and heel-toe substitutions, `\ltoeheel`, `\rtoeheel`, `\lheeltoe`, and `\rheeltoe`. They are influenced by the `Toe_heel_engraver` in the same way as `\rtoe` and siblings. Toe and heel glyphs have been harmonized in size to make these substitution marks look good.

  >     music = { f'4\rtoeheel f'4\rheeltoe
  >               f'4\ltoeheel f'4\lheeltoe \bar "||" }

  >     {
  >       \music            % #'default
  >       \set toeHeelStyle = #'standard    \music
  >       \set toeHeelStyle = #'reversed    \music
  >       \set toeHeelStyle = #'below       \music
  >       \set toeHeelStyle = #'circleheels \music
  >     }

- `NoteName` grobs are now horizontally centered by default.

#### LilyPond Changes v2.26.0 (stable-branch).

- Major changes in LilyPond
  - Notes for source compilation and packagers
- New for musical notation
  - Pitches improvements
  - Rhythm improvements
  - Expressive mark improvements
  - Repeat improvements
  - Editorial annotation improvements
  - Text and font improvements
- New for specialist notation
- Miscellaneous improvements