Support sus4 chords in root-finding algorithm

[jacobtylerwalls]

Motivation
sus4 chords are common enough that we could support them in the root-finding algorithm.

Feature summary

>>> from music21 import chord
>>> chord.Chord('F B- C E-').root()  # expected F
<music21.pitch.Pitch C>

Proposed implementation
rootnessFunction() might possibly prefer adjacent values (e.g. having a fifth and seventh trumps having a third and a seventh), but this would need to be tested (makes less sense in the case of adjacent ninth and eleventh, etc.)

Intent

[?] I plan on implementing this myself.
[ ] I am willing to pay to have this feature added.
[x] I am starting a discussion with the hope that community members will volunteer their time to create this. I understand that individuals work on features of interest to them and that this feature may never be implemented.

Reported on m21 list Sept. 2023