[color-chart] Editable color charts

[DmitrySharabin]

As @LeaVerou mentioned, it would be nice to be able to drag the colors to tweak them in that coordinate (e.g., if a certain attribute is set, e.g., editable).

[LeaVerou]

A low-fi way to do an MVP that would be to make them focusable and implement up/down arrow keys (which is needed anyway, even if DnD is possible, for a11y)