Add support for 0-fret open strings (0) AND muted open strings (X)

[KCarlile]

Background

For open position chords or open position scales, users may need to denote the option for an open string (0) or muted string (X), so the platform should support those use cases. Examples:

Am chord

X0   0
------
    1
------
  22
------

C major scale

X 000X
------
    1
------
  22
------
 33
------

Acceptance Criteria

• 0-fret markers are working ✅
• 0-fret markers can be 0 for open or X for muted ✅
  • Alternately, any character can be added to the marker just like any other fret in case someone wants to use scale tones instead of fret numbers ⚠️
  • Going with this option as it makes more sense for flexibility

Approach

• TBD

Notes

TBD

[KCarlile]

This was completed in commit 5688388.