add root mode for voicings

[felixroos]

it is now much easier to play voicings in root position:

n("0*[1 | 2]")
  .chord("<C F G C>")
  .mode("root:D2") // === .mode("root").anchor("D2")
  .voicing()
  .s("gm_acoustic_bass")

before, a similar thing could only be achieved with:

let chords = "<C F G C>";
n("0*[1 | 2]")
  .chord(chords)
  .mode("above")
  .anchor(chords.rootNotes(2))
  .voicing()
  .s("gm_acoustic_bass")

this is not only very complicated but also has the limitation of the rootNotes function, where each root just gets the octave 2 so c is always the lowest note and b is always the highest, whereas the new way will pick the root nearest to the anchor.

additionally, anchor now also understands numbers, so automating it is much easier:

chord("<Dm9 Gm11>/2")
  .segment("<2 [4 | 8]>")
  .clip(.5).release(.2)
  .dict('ireal')
  .anchor(saw.range(70,90).slow(8))
  .voicing()
  .s("square:4")