Add a chord picker

[4lejandrito]

Things that definitely need to be improved:

• [X] Add chord picker modal based on tonaljs/chord.
• [X] Clearly show muted strings.
• [X] Make it easier to select common chords (major, minor...). The list of types is too long now.
• [ ] Exclude impossible chord positions for 5 fingered humans.
• [ ] Include chords that don't start with the root.
• [ ] Include flat roots.
• [ ] When a flat chord is detected the UI shows wrong notes.

Things that would be nice to add:

• [x] Shortcuts for common chords? See https://github.com/4lejandrito/react-guitar/pull/1.
• [ ] Shortcuts for 7th 9th, sharp and flat chords.
• [ ] More settings to filter the type of chords to show (bar chords, open strings...).

I've marked the new guitar-fretter package as private. I will:

• [ ] Make it public when the logic is more polished.

[4lejandrito]

I've added a chord picker in https://github.com/4lejandrito/react-guitar/pull/10. I'll keep this issue open since I consider it a work in progress.